remember my friends 25-30psi will only be in the mid range, to remain "safe" tapering the boost back down to the 20psi range by red line is a must.
For example (RUFF example don't quote these #s they are just an example, stock SR with S15 turbo) at say 4500 rpm and 20psi the shaft speed would be ~100k rpm but by 7k eng rpm and 20psi the shaft speed will increase to ~130k rpm (engine is moving more air the turbo must work harder to keep up)
At 4500 eng rpm and ~28psi the shaft speed will be ~130k rpm, by tapering the boost back down to ~20psi by 7k eng rpm you can maintain a similar/safe shaft speed. As apposed to forcing ~28psi at 7k eng rpm causing the shaft speed to sky rocket to 150-200K rpm + of course trying to hold to much boost all the way to redline with a small turbo will also cause intake temps to sky rocket
Don't get me wrong this is not easy on a turbo but if you keep a close eye on everything and don't get greedy it is a great way to make some extra torque as long as your fuel can support it. (high octane to avoid detonation)
Unfortunately i will not have a shaft speed sensor on this turbo to show you all exact #s but this should give you an idea.
I will however be happy to post up AIT, boost, rpm data to help shed some light on this.
Don't get me wrong this is not gods gift to tuning small turbos but it does work quite well its just a matter of finding the "sweet spot" which very well may be 25psi tapered to 18psi.
haha notice i did say 25-30 yet 30 is the only # anyone repeats