Well, anything more than about 350hp and you should be considering some type of programmable EFI setup so the motor can live and perform at its best (at about 350hp you are giving up about 40hp by using factory electronics). Now you can run anything from a $400 EEC Tuner (modifies factory code) up to $3k FAST. That is up to you. You could probably get by with an SDS system and thats pretty cheap.

Lag can be tuned with the turbo you use, but you should be seeing some type of boost by 2500rpm, which you should be able to maintain during a run.

I don't know what you want for a tranny, but the 4 cyl T5's have very short first gear ratios (some are over 4:1). You would be far better off with a V8 T5 (either the 2.95 stock or 3.35 Z spec) and all that is required is either machining the input shaft slightly smaller or putting a different pilot bushing in the crank that is made specifically for the purpose.