no don't do this, leave the shims stock. Reducing the shims will decrease the squish band and lead to the plug breaking and pitting your head button and piston.

if you want more RPM you can dremel the top edge of the exhaust port just a fraction, i think adding shims will increase RPM, but i'm not 100% sure about this.

i wouldn't be too concerned with RPM, it isn't a measure of power so to say, torque is a better attribute to have as it allows you to gear for top end, whilst still having punch out of the corners.

Turque is an atribute of a lot of factors, so it isn't as easy as a shim here or there. I've fooled around with raising the sleeve height...but i have no idea what this does