Your question made me curious so I did some research:

O.S. .75 AX ABL Engine

SPECS: Displacement: 0.75 cu in (12.3cc)
Bore: 1.02" (25.8mm)
Stroke: 0.93" (23.5mm)
Practical RPM: 2,000-16,000 r.p.m.
Power Output: 2.4ps/15,000 r.p.m.
Weight: 20.39 onuces (578g) w/o muffler
26.46 ounces (750g) w/ muffler
Recommended Props: 14x6, 14x8, 15x7
1 ps = 735.499 watts, 2.4px*735.499 = 1765.1976 watts output at 15,000rpm




Tower Hobbies .75 Ball Bearing ABC Schnuerle & Muffler

SPECS: Displacement: 12.21cc (.75 cu in)
Bore: 26mm (1.02")
Stoke: 23mm (.91")
Horsepower: 2.2 BHP at 15,600 RPM
Weight: 652g (23.0 oz) (with Muffler) (jl10/00)
Weight: 535g (18.9 oz) (without Muffler)
Recommended Props: Break In: 12x6 or 13x6
1 BHP =745.7watts, 2.2BHP*745.7=1640.54 watts @ 15,600 rpm


So if you look at pure watt per rpm efficiency the OS is 10.64% more efficient than the Tower motor.

@ 13,000 rpm that is a difference of 162 watts. The OS will spin a larger propellor which may be where you will see the advantage of the 10% greater efficiency. Not to mention those large props won't fit between the booms of the Bobcat. In this case I seriously doubt you will see the benefit of your extra $100 bones by buying the OS, the OS is also heavier than the Tower motor which means you will need more nose weight and make wing loading that much higher.

I would like to see what the Tower .75 would do with a tuned pipe on this plane.