RE: 73" yak evolution 26gt
Performance with the pipe will be substantially better than with the Pitts Evo muffler as used in the QQ movie. As you can see in that movie, he also uses the baffled air entrance.
The APC is heavy, and too small. Use a lighter wood prop of 18x6. This should still spin the engine in excess of 9000 rpm and make plane response to throttle more lively due to better thrust figures.
I dislike the gear durability of the HS81. Use HS81MG instead, or you may strip gears sooner or later. Using a springy rod (like Sullivan Nyrod) or pull system also will prevent this.