Yeah I would definitely measure first.
I know this engine can run 40% nitro safely, so I figure there is room to increase compression a bit to work with 15%-25% fuel.
ORIGINAL: canardlover
RVM - before doing anything please try to measure CR first on the engine by filling the "head-space" with oil holding the piston at TDC and both valves closed. Fill to the lower part of the glow plug thread and then make your calculation.
My guess is that your engine is already optimised for the nitro contents you mention - generally products are made primarily for the US market and you are "nitro thirsty" over there....
I have no experience with a shim or gasket under the cylinder/head unit but assume it should be no problem to add one if need be....Cheers/Harald