The thing is... unless you stick a LARGE engine on, which adds weight (and the thing likes to be nose-hevay anyway... so it does NOT need it for ballast) It isn't going to fly faster with a 10X6 than it will wth the 11X4... except in a vertical dive. Its just too draggy.

You stick on the oversize engine and you lose its sweet low speed performance it was designed for.

You want speed in an inexpensive .40 size plane... Stick a 8x8 on a .46 and mount it on a Sig Kougar. (there are a lot of others that would be good candidates for speed too.) But you're going to need a good long runway to give it time to build speed with the short high pitch prop.