Except for the extractor spring, buffer weight and possibly gas port size, PSA kits usually work well enough. Replace the PSA extractor spring with a Colt spring and the carbine weight buffer with an H2. A BRT gas tube can fix over gassing (if any).Looking at PSA lower pistol kits and they seem to have a "KAK Industries Shockwave Blade" as an arm brace.
Anyone tell me about those, as in what's good about them and what is not so good about them?
My shorty gets shot a lot and the Shockwave is very narrow and concentrated recoil into a small area. The SBA3 is much more comfortable. It has more surface area and more comfortable to use. I install a Split Fix strap to keep the rubber "wings" from folding or collapsing. The Split Fix is longer and wraps around the arm better.
10.5 I know works in LEO carbines OK fine but the 7.5 is three inches shorter and so maybe more easily used in tight places, though I am concerned about even more velocity loss.
I've done some shooting 5.56 with 10.5" barrels and 11.5" barrels, suppressed and unsuppressed. I much prefer the 11.5". The 11.5" gives more velocity and has the edge in performance out to 300 yards or more, is a little more reliable and a bit more forgiving. A lot of shooters prefer the 12.5", claiming it's in the sweet spot between 10.5" and 14.5". I've not had the chance to put one through the wringer.
My best shorty setup is an 11.5" barrel, BRT Micro Port, A5 RE with an A5H2 buffer, Sprinco green spring and Omega direct thread suppressor. (I have a BRT 11.5" OPTIMUM SBT Suppressor Barrel 5/8-24 Tapered Shoulder that I suspect I'll like even better once I get the upper built.)
A suppressed shorty may be as long as a carbine, but it's much quieter than an unsuppressed 20 inch. If you don't want or can't have a suppressor, look into the BRT Covert Comp. It's an inline Comp that's about the same size and weight as an A2 birdcage but does a good job pushing the concussion and blast down range. It won't make your shorty any quieter, but it will make it more pleasant to be around.
My next bit of advice is to stop dithering and get busy collecting parts. If you buy kit & lower from PSA, you'll save on shipping charges.