When I started in STS I didn't worry about having the lightest car or making the most power. Instead I focused on getting the most practice and improving until I got to the point that I was within tenths of beating the fastest other STS guys at big events. That would be my advice to you if you want to improve.
To answer your specific question you are allowed to overbore the piston sleeves to the first limit listed in the Mazda service manual (most likely .010"). Additionally you can resurface the head a tiny amount. The combination of these changes raises the compression ratio about .1 point and together are likely only worth 2-3hp. You aren't allowed to make any other changes.
If you are getting transmission grinds that would be a transmission problem not a clutch problem.