A much better way to reduce compression is to make the head button bigger. Adding too many shims will make the distance from piston to the squish band too big and will also cause detonation or rather pre-ignition. A reason for that is that the heat transfer from the hot piston to the cooler head will decrease with a bigger gap, making the fuel ignite too early. A big gap will also reduce squish velocity and reduce power. Aim for a gap of ~0.4mm for a .21 engine.
You can easily run 9.5:1 compression with 40% nitro or higher if your engine is set up properly.