First, realize that with a single zone system and multiple floors, it's difficult to get the temperature correct everywhere. Also, since you mentioned this was an attic space, you're likely against the roof and getting heat from every wall plus from the computers and people in the space. In short, you're fighting a losing battle.
I've got a similar challenge, though not to your extreme, and here's what I have done. First, learn how to adjust the baffles on the ducts. In the summer, you want as much as possible going to the high floors, and the reverse in the winter. Get someone to feel the air coming out while you're pushing the baffle all the way to one side or another. For me, it made little difference until I pushed it all the way.
Next, exclude any rooms you don't need to heat/cool, e.g. a guest room or storage space in the basement. Shut the door and close the vents to reduce the effort you're putting on the HVAC.
Finally, I swapped out my return grille with models that accept a filter (they are hinged and open after releasing two small clips). These return grilles are special orders from the big box stores, but they carry them. I still leave a thin blue "rock catcher" filter directly on my HVAC blower itself. And I put a filter in every return except the upstairs in the summer and downstairs in the winter. The idea being to suck out as much air as possible from location that needs it.
This means that switching between heat and cool is a process, flipping the baffles and swapping in and out filters. But a little effort is better than a lot of sweating.
One last suggestion, since it sounds like you're running a small server farm in your attic. Either move that down to the basement, or consider replacing the window unit with a ductless mini-split system. You get to see out the window, and these systems have configurable thermostats.
I have a couple of long runs too and I used a fan until I upgraded the HVAC unit. Before you get a booster you need to measure the air flow out of the two vents and compare them to other vents in the house.
How to measure air flow? Well pick up an Air Flow Meter. However I have a suggestion before you spend $$$ on this. Get a large trash bag, configure it to open about the size of your duct opening (take off cover), hold it tight on the duct opening, measure how long it takes to fill... compare several vents. Note the air should be running when starting this - since how long it takes air to come out factors very little into the cooling equation. If you are confident with the bag results and they are obvious then skip the pricy tool. ##Side Note## (A friend of mine uses one of those garden wind spinning things and measures the rotations over a given time to compare air flow. You have to have the angles/distance the same from the vent and he has to have his iphone slow wdown the frame rate to count. But damn that mcgyver-bastard, pretty sure its more accurate than my empty trash bag! But it does involve slow-motion so my solution is more practical)
I would also take temperature readings inside each duct while on after 5-10 minutes. How cold is the air you are getting?
If you are getting comparable air flow and temp you can add another duct or bigger. Really up to you.
If you aren't and I am guessing you aren't then you need to first get rid of the flex lines. They squash air flow, they are affected by outside temperatures more, and they leak. Put rigid metal ducts in. If the ducts are in the attic then you need to insulate the crap out of them.
Then if you still have issues you need to measure air flow and temp again. If air flow is poor you are looking at a booster. If it is relatively the same, add another duct.
Then you are talking about adding a return to the room and over-insulating the attic above the room.
Best Answer
The first step is always to lower where the heat escapes into the house (for summer cooling season). You most likely have plenty of attic insulation. But you might check for other air leakage places (around doors, windows, pipes, electrical outlets, etc.) Less heat entering is less heat to cool. Shades or solar film over windows help too, as well as trees etc.
If the air ducts are insulated, then there might be some difference in moving the duct work to the crawl space, but most likely not enough to cover the cost to change it. If they are not insulated, then the cost of installation still might be higher than many years of savings.
Another possibility for savings might be an attic vent fan, which could lower the attic temperature some. Just lowering the attic temp by 10 or 20 degrees is worthwhile in many places. It means a little less heat getting through the insulation and the ducts would absorb a little less heat as well.