I can attest to using the factory a/c setup. I've been running a much simpler setup for about 2 years now with great success. I was working on building a stand alone system for vehicles, but I was hindered by supplier issues and more importantly weight. I just don't think its feasable to carry around another cooling system when you already have the factory one right there.
Anyway, I borrowed the idea from a guy on here named Duner. I'm not sure if he hangs around much anymore, but nevertheless it works. The downside is you lose your heater, but in the summer who really cares. I never use my heater here anyway, so I use the setup year round. Bascially you have an a2w IC, with a pump and everything, and then you route the coolant lines to the heater core. When you turn on your a/c, you set the **** to heat. The fan then blows the air across the evap coil onto the heater coil, thus pulling heat out of the water. It takes longer to cool off the cab, but like Rick said, below ambient IAT's ALL DAY LONG! Even under boost. At idle in the middle of the summer (+100* ambiet), while stuck in traffic, IAT's would be a little below ambient. Step on the gas to go, and they drop instantly.
At the end of a 1/4mi run they start to climb, but that probably had to do with my lack of reservoir capacity. I never played with pump speed, but I'm sure there's more to be had there as well. An evap coil submerged in water would be even more effective IMO, but I worry about what it'll do to the in cab temperatures too. You might have to play with the orifice size and the amount of refrigerant. A variable orifice for sever conditions would probably be the best way to go.
I know that with no reservior, and driving it really hard, like a full 1/4mi it would be blowing luke warm air. Not hot, but not cold either. I've never measured the air temp across the heater core though, just the IAT's.
It's really quite interesting, because as soon as you get into boost the IAT's drop further from where they were while cruising. With meth and chilled water, on a 90* day, my IAT's were 50* at the begining of a run and ~65* at the end. That's with 10-12psi on a 4.8. I shut the a/c off in stagging too, since the IC and water will stay cold for quite awhile depsite the a/c running.
I think I'm going to due something similar to my Ty since the IC sits on top of the damned engine.