I've been meaning to comment on this beauty...That's a nasty looking bird..!
I don't know about the LA .25s, but the LA .15 is pretty hot with the right prop. It can handle 18,000 all day.
If the LA.25 is OK at 18,000 then I wouldn't be bashful about going with less D and more P.
The RC Combat guys would know about the LA .25's limits.
7 x 6 would be my starting place if looking for speed.
APC might still carry a 6.5 x 6.5 and if you could carve a reasonable facsimile of a pusher....
There IS something cool about running a prop that you carved. ..but it's also kind of eerie to stand next to.