Brian,
If you want to use a Jett muffler, the one that fits the Jett .50 will also fit the .HB .61 PDP. I lost my stock muffler in-flight and when searching for a replacement, I discovered the muffler off my Jett .50 fit perfectly. I did have to open the through holes in the case ever so slighly since the stock threads of the HB are 5-something and the Jett is tapped for 6-32.
I'm spinning a Vess 11-7 at 13,300 on 10% Wildcat with extra castor added. This equates to almost 1.8 HP on Pe Reivers HP calculator. That's about an 800 RPM boost over the stock muffler. I don't know how that compares to an OS .61 FX, but it's quite respectable.
I don't consider my HB .61 PDP to be any more "thursty" than any of my other engines. It really moves my Komet. Unlimited vertical and VERY fast straight and level flight.