Is a smaller pulley available for your setup? If so, you could probably get it to squirt out some more PSI, but your blower might not last as long. If you have a Powerdyne unit, I suspect it's already maxed out at 6psi.
What's your air intake like? What's your fuelling system like? Might be time to upgrade to a larger blower.
As for the head gasket, an AU steel gasket should do the trick, but unfortunately since it's thinner it'll add a little to your motor's compression :(
The T5 would only handle 250rwkw is treated really gently, normally I would say 200rwkw would be about the max you would want to put through one.
As for the head gasket, just make sure you get the head surface machined and use a good quality gasket like ACL etc. The AU one is good but its too thin for forced induction. What are you using to tune the engine and what are the AF ratios like? With a decent tune you shouldn't have any real issues with head gaskets and a blower, even if the comp is a bit high.
While you are at it, talk to Wade Cams about a blower specific cam.
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