It's doubtful you will see much power difference by adding head shims and nitro vs. using low nitro and stock head shims. Downunder says he shaves the head to raise the compression when using FAI fuel and can get as much power out of his engines as those of us using nitro.

Personally, I'd go the cheaper route and run lower nitro while leaving the shims stock if at all possible. For me, cheap is king!

John