There isn't a lot of stuff you can do to an N/A car to make it go fast. I think the fastest N/A cars top out somewhere around 240 whp.... with cam, heads, headers, etc. If you want to go faster, you'll need some sort of forced induction... top swap or turbo... a top swap would be simpler from a tuning standpoint and there's lots of L67 parts laying around so it would be cheap (around $500), but a turbo has much more potential. Until you cook the diff, that is.