James:

The horsepower figures given by engine manufacturers are pure hype. They bear no relation to reality. I have the TT 120, the OS 160 and the OS 91, and I can tell you that in the real world, the OS 91 and TT120 are very close in usable power. The OS 108 (which I have flown in the past) is a torquey old chunk and will pull a big prop better than either of them. The OS 160 is in a completely different category. It probably has 50% more usable power than the TT120.

BTW, the 91FX prefers a 14x6, the TT120 likes a 16x6 and the 160 turns a 17x6 so fast the prop tips go supersonic. Here in the US, the 160 is clearly the two stroke of choice for 120 size aerobats. (All props are APC)

You might also want to consider the Super Tiger 140 (2300). It offers a higher power to weight ratio than even the 160 and turns a 17x8 close to 10,000.

Don't be overly concerned with the weight. In reading the threads it sounds like your Extra has a tendency to come out tail heavy. You will hate it if you sacrifice power for weight and then have to add lead to the nose to balance the airplane.

Jim