Is this a modern 25 with a baffled muffler?
I ask because I've got a 15LA and an old Enya 15. The old Enya is a fair bit louder as it just has a strap on with no internal baffling.

BTW, remember that DB is a logarithmic scale, so 10-12db should make quite a difference.

A typical 25 would normally run a 9x6, as a trainer doesn't need to be fast, you could try a 10x4 or 10x5. If the engine is powerful enough for the plane you'll be chugging around at half throttle or less anyway once you got to altitude, so the amount of noise you make has probably got more to do with your flying style than the engine.....

I'd try it, then once you know how it flies and how noisy it is, you can experiment accordingly...