Perhaps, but the disc area close to the hub isn't doing much work anyway. Reynold's numbers increase as you move outward. You could also imagine that reducing the turbulence/drag around the hub area and fairing the power system into the fuselage to reduce drag coefficient is of more importance than using the inner portion of the prop.
On that note, that poses an interesting question. Has anyone ever quantitatively (it took me three trys to spel that rite..
) measured an increase in rpm (or the opposite) between no spinner and a balanced spinner of sensible size? Hmm.
The percentage of pitch speed you achieve, or efficiency as you termed it, is so-o-o dependent on the airframe. So you might achieve 50% on one aircraft while achieving 85% on another. It is not unheard of that the airspeed on highly powered aircraft exceed these theoretical numbers (in fact, this has been specifically reported with TT .46's and 10-7 props.. by airraptor IIRC). This could easily be from unloading in the air, but also because there is more than one way to measure pitch - the pitch from the lower surface datum line is always lower than the aerodynamic pitch for example. Roughly the same deal as the zero lift angle of an airfoil is often negative in relation to the chord line.
In the end anyone will tell you - use these napkin calcs to get you in the ballpark, then experiment to find the best combo. There are a lot of variables at work simultaneously.
I'd still put about $3.50 (but no more..) that a 10-8 or maybe 10-9 might be the quickest. If the drag is low enough then the higher pitches will win out. It will be interesting to see.
Or as speedracer says, maybe 11" props work the best!