You are absolutely and entirely outside rules territory from the moment you've decided to fire someone out of a trebuchet and into the ground, so only loose inferences from the rules are going to help us here. But let's see what it gives us to work with.
Forewarning: if you're after real-world simulation in D&D 4e, you've come to the wrong town. The designers consciously eschewed real-world simulation in order to just make a fun and well-balanced game, so 4e runs on simplistic game logic that laughs in the face of our Earthen physics. 4e has squircles for instance: consider that the circle is defined as a shape where every side is equidistant from the center, and that this perfectly describes a square room of any size as far as a walking PC is concerned. Instead of attempting to draw real-world physics out of D&D 4e—which 4e will either directly contradict or simply not help you with—use the game logic, leave the calculator aside and use your time to beat stuff up and do fun things instead.
this would launch him at about 3X the speed of a normal fall (150 feet per second instead of 55 feet per second).
You have some parts of this conclusion wrong.
First, there's nothing that makes him fall faster. You have a trebuchet, but we don't know how fast he's actually traveling: D&D 4e doesn't exactly have specific rules for trebuchets that I'm aware of, unless you'd like to point us to them.
Second, the speed of a normal fall is 500 feet per round, at the end of the creature's turn (Rules Compendium, p209, High-Altitude Falls). Since a round is 6 seconds, that's actually 83.33 (recurring) feet per second, and nobody ever falls faster than this. That's far less than the falling speed of a skydiver (who is trying to fall slowly), so we can assume accurate simulations of Earth physics and wind resistance are to be left well aside: things work differently here.
So. Your orc hits the ground normally, and takes the normal amount of fall damage.
If they hit the wall instead, we don't know if they're really traveling fast enough to even simulate it with an equation that works from 500 feet/second of fall damage. You're in house rule territory, and we've established D&D 4e doesn't really care about what physics has to say about falling, so make a judgement call with a small portion of fall damage, or decide your character survives it fine because they might as well.
Alternately, whilst I would not advise against firing D&D 4e characters out of trebuchets on account of how awesome that is, I would at least just advise saying they succeed and skipping this trouble.
Unless they're an enemy NPC, in which case I advise they die hilariously.
Rules
From the Player's Handbook:
High Jump. When you make a high jump, you leap into the air a number
of feet equal to 3 + your Strength modifier if you move at least 10
feet on foot immediately before the jump. When you make a standing
high jump, you can jump only half that distance. Either way, each foot
you clear on the jump costs a foot of movement. In some circumstances,
your DM might allow you to make a Strength (Athletics) check to jump
higher than you normally can.
p. 182
Falling
A fall from a great height is one of the most common hazards facing an
adventurer.
At the end of a fall, a creature takes 1d6 bludgeoning damage for
every 10 feet it fell, to a maximum of 20d6. The creature lands prone,
unless it avoids taking damage from the fall.
p. 183
Step of the Wind
... Your jump distance is doubled for the turn
p. 78
Jump
... the creature's jump distance is tripled ...
p. 254
Combining Magical Effects
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect - such as the highest bonus - from those castings applies while their durations overlap.
p. 205
This last gives several interesting possibilities for how these combinations actually work - but that is another question.
Definitions
Giving "jump" and "fall" their common English meanings (1.b & 2.b being the most appropriate), they are not the same thing. Jumping is voluntary and in control, falling is involuntary and out of control. So the first does not automatically lead to the second to my mind.
Designer Commentary
In addition there is this quote:
@Plaguescarred Can you jump farther than your movement when using
magic i.e spell Jump & boots of striding and springing?
@mikemearls
i'd rule yes - design intent is to make you jump super far
Read more: http://www.enworld.org/forum/showthread.php?362742-Jump/page2#ixzz3SirpzT8U
So we can draw two things from this:
- It's at DM discretion (what isn't)
- There's magic involved.
Discussion
Naked Jumping
Starting from the basics, a PC can jump up 3 + Strength modifier, without magic this is a maximum of 8' - so there are no falling considerations. A monster or magiced-up PC with a Strength of 24+ or a thief with Second Storey Work and high enough Strength and Dexterity can jump into the danger zone (>10'). I would argue that if you are strong enough to jump that high, you have the muscle strength to absorb the shock of landing without damage. There is some simple physics involved here - the speed at which you leave the ground equals the speed that you return - if your leg muscles can generate that speed then I say they can absorb it.
As a side note, I would rule that you can safely jump down less than 10' or equal to your maximum high jump whichever is the greater. Oh, and they wouldn't need a run to jump "straight" down. If you are jumping across a chasm to a lower ledge, I would rule that the so long as the total drop from high point to low point did not exceed this falling would not be a consideration.
Step of the Wind
A monk with Step of the Wind can double these heights. A 5th level monk wouldn't take damage from a fall like this anyway. For lower levels, as this is a specific class feature with a cost; I would be disinclined to nerf it.
Jump Spell
Jump can be cast on anyone and takes the maximum of a very strong PC to 24' and monsters to 36'. I cannot accept that this 1st level spell is intended to give the PC the power to jump up in the air (to whatever height) and then come down by falling to their death. This is simply not credible, particularly when you take into account the designer's comment on how he sees the spell being used.
The Combination
Wow! What a great idea the PC had! Of course you don't take falling damage and have some inspiration while you're at it!
Best Answer
While it's not spelled out explicitly that you take no damage, the intention seems to be that you take no damage.
You fall 30ft per round, which is about 5ft per second, which is much too slow to be taking falling damage. (Even slower than Feather Fall, which is 60ft/round)
Keep in mind that you're descending at a regular walking pace, and you are free to land on your feet at the end.
Do keep in mind that the "slow fall" feature only works while you are using the flying option of the boots. If you just drop off a cliff without flying, the boots will not protect you.