I am pretty sure meth cools down your intake air so it would get more hp. The problem is that the stock SR turbo, even the S15 gt28r, only holds up to like 18-20psi. Compare to EVO and other newer car with turbo (seen them with over 23psi all the time), they need them more.

Tune your car, turn up the boost, and you should make over 350whp.