S15 turbo is old. And weak.
It was still good in like 08'
Nowadays, you can get a 'larger' turbo that flows wayyy more.
for example a couple months ago
dynojet roller hit for a 500bhp turbo, with the usual 4400~rpm range of spool. The SR20 goes to around 7k and does 140mph in 1:1.
You can see the peak torque (max boost) is way before 5252rpm because the horsepower always crosses torque exactly 5252 (which is 33,000lbs/2*pi feet)