According to the core rules (and I checked for errata), a psyker "...makes a Power Roll by rolling a number of dice up to their Psy Rating (typically 1-6) and adding their Willpower Bonus."
The important phrase here is "up to"; just because a psyker has a psy rating of 4, that doesn't mean they have to (or should) be rolling 4d10 for every psychic power they try to use.
(Note that there was errata that you always have to roll one die, which you wouldn't want to do if, for example, your WP bonus was greater than the Threshold of the power you wanted to use.)
It's also worth noting that rolling 9s isn't a guaranteed path to INSTANT TPK - it just invokes Psychic Phenomena, which CAN result in Perils Of The Warp, and THAT's when things get Bad. (It's also worth noting that results of Psychic Phenomena or Perils Of The Warp do not cancel the effect of the power, unless otherwise stated; you've still rolled a 9, and stuff gets weird, but you also got a pretty good result on that die!)
Finally, the "Favored By The Warp" talent allows psykers to roll twice and choose the better result on all Psychic Phenomena tests; if a psyker opts to enter the Scholar line of ranks beginning at 2,000 XP, he can pick it up at Scholar Arcanum (6,000 XP) for 200 XP. If he opts for the Savant tree instead, he cannot acquire it until late enough that Ascension advancement rules kick in; if your Psyker is still coming close to blowing up everything but has opted to become a Savant, consider letting him take "Favored By The Warp" as an Elite Advance after 6,000 XP.
You've made two key changes:
- Allowing multiple combat half-actions per round
- allowing movement to be replaced by a non-movement half-action
The primary factors I see are:
- increase in Psyker offensive capability (Most of their offense is more powerful than their allowed weapons; it gets worse under RT or DW, due to a different psychic system)
- decreased access to defenses¹
- decreased emphasis on movement²
- increased emphasis on non-action defense (IE, cover)³
- breaks the equipment bonuses
- decreases incentives to use burst and full-auto autofire modes
- cybernetics and talents with extra half-actions become devalued⁴
- initiative becomes more important⁵
- Ganging up becomes less essential⁶
¹: Since one gets very few defenses per turn, this change of yours makes them less valuable, by allowing them to be overwhelmed faster.
²: The current system makes movement a valuable and essentially irreplaceable part of the round; take it or not, it's lost if unused or unusable. It's not terribly realistic, but is very cinematic. And DH, RT, and DW are all intended to be very cinematic in tone.
³: At present, especially for melee, active defenses (dodge and parry) are quite valuable; cover is less so, but not enough to render it useless. When one allows replaceing movement with a second attack, cover suddenly becomes MUCH more attractive, as it's an assured penalty to be hit, instead of a roll by the defender... but it also reduces drama.
⁴: At present, the only way to get a second attack is expensive talents and/or cyberware with inexpensive talents. If a character can just hole up in cover and double shoot, these talents are far less valuable, making the Assassin and Guardsman's additional combat actions less useful.
⁵: Further, by allowing the etra attacks, you can run a target out of actions sooner, as well, so initiative becomes more important to prevent having actions drained in defenses.
⁶: Ganging up is the great equalizer in RT and DH combats; it's how one overwhelms defenses. Given that every character can defend once for free, and once by aborting, if you can attack twice, you've just eaten both defenses this round, meaning you only need one attack from a buddy to get through. The norm is that you need two buddies to negate the defenses, or one buddy with a multiple attack talent.
It sounds like you made these changes with rank 1-2 characters... it's going to be more profoundly off-norm or higher rank characters, as normal slow growth of actions is made to feel even slower by providing far less of a bonus over not having them.
Best Answer
You can Focus Power once per Round
Per the Dark Heresy 1st Edition errata, currently located at https://images-cdn.fantasyflightgames.com/ffg_content/dark-heresy/pdf/darkheresy-errata-v3.0.pdf
So no matter what, you can only ever perform the Focus Power Action once per turn, as it counts as a Half Action Attack (even if the power isn't an attack), and there are already rules in the Actions section regarding the inability to perform multiple Attack Actions (don't have my book to properly cite this, but the errata clearly states only one Action per Round).
The change in the errata was made to prevent Psykers from using both Focus Power and Half Action Standard Attack Actions in the same turn. The Action section in the combat chapter of the core rulebook (specifically under Half Actions) should have some sort of wording that states you can't perform the same Half Action twice in a round. You can't Standard Attack twice for example. You technically are taking a separate Full Round Move Action, not performing two Half Action Moves.