Stay away from the Variable Turbos that Gm uses (Garrett), that turbo spools slower than molasses in january. Even though they are supposed to spool quicker.
Best bet is to find a Ball Bearing turbo. IMO the best BB turbos are the Turbonetics Ceramic Ball Bearing turbos. But they can be expensive as is every other BB turbo.
Your best bet is to just go with a bigger one while your at it. You may not think you need it down the road but later on Im sure youll want it.
My question is why do you want faster spool but no more boost?? The stock LB7 turbo spools pretty darn quick. Granted you cant mash it from idle and expect it to shoot up instantly but lets be realistic no turbo will do that.
For a cheap quick spooling turbo BDs Supermax spools quick and is cheap.
MPI twins are also and option, they are about $4500 and utilize your stock turbo and a BIG turbo. Theyll give you PLENTY of boost.
Like I said, just go with a big one, youll thank yourself in the end.