I haven't tried the N of 9x7.5 but have tried the 8.8x8.25N(as far as i can remember) and I think it required a lot higher rpm engine.
the one on the plane was 9x8, I would go with 9x75 next. as the engine gets older 9x8 is a bit too much.
So far with OS 46 ax 2 we have used 9x7.5 9x8 and 10x6.
the 10x6 is surprisingly good on some planes we fly here.
as for the plug I am using just os 8. What is the difference between os 8 and merlin on RPM?
You can try dubb jett for merlin plug.