Anyway, to be honest, not much. ECM modification and tuning usually has more gain when there are modifications to the engine that would benefit from tuning. A stock Vortec engine isn't going to gain much from reprogramming the ECM, but with headers, roller rockers, a cam swap, and other minor bolt ons, you might notice a 15hp gain from the ECM over the other parts, maybe more, but not much.
I personally couldn't do @#$% for ya, I don't do the programming. Try Fastchip, they do most of our work and they have a lot of experience with the GM FI motors.