RPM's don't keep a turbo spooled really. To get boost, you have to have load , ie: be accelerating.
Depending upon your turbo setup, the time from flooring it to boost is probably near to same at 3000 as it is at 5000, because the turbo is just goin' wit' the flo' unless the engine is under load, when it starts spooling.
The reason I say nearly is there will be some small variation, but at higher rpm cruising, the turbo may be spinning faster in cruise, but it has to speed up more to produce the same amount of boost as it would at lower rpms - pretty much a wash.
I stated previously that I wanted a real long rod 302 and turbo it. I still say this because I want something quiet and tame to drive daily, but when I stand on it I want to GO! I don't WANT a lot of off-idle torque that ci's give you...so stick with your plan...