Obviously i use the GT Commando 2; I have an Armson 16 in Stealth barrel, 20 oz CO2 (about to get a 48 ci 3000 psi nitro)
Definitley the barrel, and anything you can do to increase the accuracy of the gun. Nitro would be nice. Also, any sight rail you have could help (I dont know; I've never used a sight.
It has a lot of things on it that you normally wouldn't get off the bat from other markers (the sight rail, an awesome stock, the ok barrel, the expansion chamber, and an over all cool look). I was using a case of RP All Stars- 500 rounds, playing with my friends. It was in the lower 40s that day, and most guns will get a lot of ball breakage in that weather. I didn't break one. I also just finished off a case of Brass Eagle paint ( I know its cheap, but 14 year olds don't have a great inflow of cash), with only one ball break.
When I first recieved it and put some paint through it, it leaked from crevaced I didnt even know existed, but mainly from the expansion chamber, the cocking bolt, and the bottomline CO2 "connector" (I dont keep up with all of the terms for parts. In lame man's terms, the place where you screw in the CO2). I sent it back to the company, and in a couple of weeks I got it back, as good as new. The expansion chamber makes it a little tricky to unscrew air, but it isnt too bad. One other thing- there are no electronic or automatic grips compatible with the GT Commando 2 (to my knowldege).
Over all I would say that this is a good gun for snipers, or those who prefer to take very accurate shots rather that "spray and pray". A speedball player may not want this gun, but I think that reasonable price probably makes up for all short falls it has.