Well guys the weather was pretty bad today at our field, definitely not weather for flying my big plane. Still I test ran the engine with the pitts muffler fitted (after removing the gutted muffler) and came up with some interesting results. I'll start with my original 25cc conversion as a baseline and then list the 30cc figures. Both engines are similar with differences noted:

25cc with pitts muffler - Best rpm with 16x8 prop 8000rpm
30cc with gutted muffler - Best rpm with 16x8 prop 7300rpm (note the only difference from the 25cc was a different spark plug).
30cc with pitts muffler - Best rpm with 16x8 prop 7430rpm (same as previous except for pitts muffler with a 10mm spacer to fit the muffler clear of magneto).
30cc with ZERO muffler - Best rpm with 16x8 prop 7610rpm (near zero throttle response, either a high idle or near full revs)

Something doesn't seem right here, I would have expected the 30cc engine (particularly the third listing) to have turned at least 8000 with a potential for 8300rpm. This is now making me think what differences there could be between the piston and cylinder (only parts that vary). I suspect the port timing to be different or that the 2nd ring on the 30cc adds some massive drag (weird).

Future testing will involve the same engine refitted with the 25cc piston/rod and cylinder. If the rpm is 8000rpm then something is definitely different with the 30cc parts, which would most likely be the port timing or transfer ports.