Spool time, DRIVER SKILL has alot to do with it. If your not making the power that you should, well , then we need to see the a/f ratio. You SHOULD be making around 350-400 give or take with those mods. I know our single turbo 2JZ makes 417whp on 15psi. Its a single with EMS, but everything else is stock, it ran 13.0 with no second gear at commerce, it runs 11.5s now (granted this is a much lighter 240sx chasis but still)

STOCK STOCK SUpras should still run mid 14s with ease. Making 350whp you should be in the 13s ESPECIALLY with a RWD car. Brake boost at the line an get your 60 fott down.