a size .40 engine can take a much bigger prop than a 9.5-5

how does it feel at the moment?

if it feels underpowered then fit a bigger prop, try and get a tachometer to read the rev's on the engine, if you are getting any higher than aout 14,000 then put a bigger prop on as if engines dont have the right load on them then they can lose life span

too fast and they wear out too quickly, too slow and they get overheated and burn up

i would put an 11-5 regardless of what the current performance is like, you'll get more economy out of your engine therefore more flight time