I've been working with gas engines for 20 years and currently have everything from 3W to DA's on hand and am very familiar with gas engines. Am currently flying a 33% Extra with a 3W100 so I am not new to the game.
Using the pitts muffler recommended by Horizon, and a 17-6 APC, I get no more than 7,800 RPM which from what I hear is low.
Sorry, but those are the facts.
At this point all I can tell you is that the tuned pipe was back ordered and will be in sometime in the next few weeks (just in time for winter winds) and when I get it, I will install it and report back to everyone.
Although the choke arrangement was not designed well, this does appear to be an excellent engine but for the fact that I can't get the RPM up to snuff. As I said, I got nothing out of their Tech Department and this irritated me some. All I got was sort of a "so".

3dbob