I would wait until it's broken in and then go with a 5% pulley. Otherwise, it's a vicious cycle...upgrade the intake....upgrade the exhaust...upgrade more pulleys...upgrade the blower...rebuild the engine - repeat.
well i have it in a 94 mustang with the mustang headers and dual exhaust so thats better than the sc exhaust and ive seen stock sc's running 14 psi with no problem
You said '1990 SC stock' - a '94 Mustang with dual exhaust is far from a stock 1990 SC. What else you hiding? Is it actually your sister's car as well?
well its a stock motor out of a 90 sc so i figured it wouldnt be a big deal that it is in a different car btw what does a stock 1990sc run on the 1/4 mile
To answer your original question, people have run over 20 lbs of boost through their bone stock engines. I don't think the gaskets last too long though. Stock sc's run between low and high 15's for the most part. 5-speeds run on the faster side of that.