reaaz:
Sorry for the delay, just noticed your post asking about YS91 being adequate.
It will probably pop, not just climb out of hover, I would guess.
I have hovered and climbed out smartly with my 6#11oz build of this plane (details of lightweighting I did here, @post #943: http://www.rcuniverse.com/forum/m_47...tm.htm#8869810

My engine is just a little ol' Saito 91, at 22oz. I am sure that you will hover with the extra power of the pump/supercharger.
You may want to consider the 14x10 for knife edge. I played a lot with props, and the only way to get my 91 Saito to climb (slightly) in KE was to up pitch it to a 14x7 pattern prop. Apparently that is a common issue with Extra airframes, the lack of side area means lots of speed to hold or climb in KE. I am guessing that your YS can turn more prop than my Saito and see 14 x 10 rec for that engine. I have run 15" props OK on this plane- they just get a little greener...!