Turning up the boost is a free option. fit a boost guage with 1psi incrememnts and set it to 9psi using a mechanical boost controller.
you can run 12psi no preoblem but it will involve canging your spark plugs and there gaps and possibly adjusting your timing. A reeer flowing air filter and removing the s12s centre silencer and changing the rear box are also cheap ways to get good power gains. a 35bhp increase over standard is quite easily achievable for peanuts
