Piet, you're on the right track. Dar's advice is good. Work out the rpm band you want to work in and tune the pipe dimensions to suit.

The OS FX .91 should easily get 2 - 2.5 hp (using the Pe Reivers spreadsheet) at rpm levels in the 9500 - 11,000 rpm zone with the right pipe and header dimensions.

That's the neat thing about sport two-strokes - they have a very wide band of rpm where good power can be coaxed from them with a tuned pipe if you're prepared to try different length pipes. I've seen an OS .50 SX put out nearly 1.5hp at a very low (for a .50 SX) 11,000 rpm on an APC 12 x 8 with a long pipe. With a shorter pipe, the same engine on the same day put out 1.66 hp at 14,300 (APC 11 x 6). Sure, this engine was happier doing 14K than 11K but if you need to worry about noise and reduce rpm without compromising power too much (as you do in pattern), this pipe tuning flexibility can be very handy.

I've done quite a bit of taching on several FX .91s and measured their power output. In stock trim, the OS .91 FX is a relatively low revving engine. It's happiest with its stock muffler running anywhere from 9500 to about 11,000 rpm. With a tuned pipe at the right length for its application, this can be extended from as low as 9K to as high as 13K or more.