Sport Pilot- it is a sad situation if an engine has to limp around at 8K for noise limitations when it is supposed to buzz at 15K. Another reason for 4 strokes!
The OS 1.4 and Webra 1.6 RE two strokes were not intended to run at 15,000! They are set up by the manufactures to run at 9,000. Even if set up for speed I doubt they would turn 15,000. The larger the engine the more difficult it is to get high revs without something breaking. Look at Dub Jett's engines. Their larger engines do not get max power at 15,000, but several thousand RPM less. Even in the 80's when pattern engines were limited to 10cc they were turning in the 9,000's with props such as 12x12 and 13x10 two blades. Two stroke engines mind you. They were getting about the same power as most of todays .91 four strokes.