Well here is my opinion.
I have tested many turbos on my car and many boost ranges. I personally reached my limit when I say that even my car can handle more but the power is enough for me.
I am currently running with a gt28rs 0,64 a/r but tried the 0,86 a/r before. It was spooling to 6psi fast as all the ball bearing turbos, but then as it was a big a/r I only had my full boost at 3800rpm. I do have a high reving car, but still found that too late, so I changed to the smaller 0,64 a/r. I am happy with it now. Both the gt28r 28rs can boost enough air till 20+psi. The rs is more efficient on higher boost levels.
Now the funny thing is I only runned my setup once at 21 psi and then I just turnwed theboost back to 14psi. I asume i had like 350whp at 21 psi. That is just not a street car anymore, that is a full race car with that or higher power. i am not that good to drive a car with that much power. if my wheels spin in the 1st 2nd and even a bit in 3rd gear with my LSD and long gearing in it, thats far enough for me to controll.
If you are planning to build a race car (just for race) then 5-600hp is ok, you dont even have to take car of the turbo lag, as on acceleration you are allways over a certain rpm, lets say you rev up to 8000rpm, so when you change gear it drops back to ~6000-6300 rpm, so it would mean it is far enough for you to get full boost after 5000rpm. on race cars they use nitrous just in the first gear to elliminate the turbo lag if needed, but then they just stay over the full boost rpm.
If you are planning a street performance car, then I say the max you would want is 400whp as high boost and 300whp max on the street.
As I started thats my personal opinion.