3 and 4 blade props aren't as efficient as 2 blades at any rpm
but in high power applications a 2 blade would be so big that the stress it puts on the engine would tear the crank apart so in that case a 2 blade wouldn't be very efficient
that is why you see 3 and 4 blades on acrobatic full scale planes and the old warbirds
on the acro planes a 2 blade puts stress on the engine from all the rapid change of directions
also as I said some pattern guy use 3 and 4 blades for the downline braking they offer
If flat out speed isn't your main goal you can try a 3 blade I run them sometimes just to do it they sound different and are quieter in most cases.