Regarding the XLS .36...I used a .272" venturi, and have used the .46LA muffler and the stock Magnum muffler.
Ran a lot of Powermaster 10-22, but also some 10-18, launch at 9,500 to 9,700. I have one in Monty Summac's old Olympic...need to get that back in the air. Needs longer LG legs for pavement! A Randy Aero CNC Tube muffler should work with pretty similar results. It worked very well for Monty with his homemade tube muffler. I used a Thunderbolt long idle bar, sometimes a Thunderbolt 4-cycle plug. The TT Cyclone 11 x 4.5 is really a magical prop for that engine. I also tried 11-4 Power Point and 11-4 MA Scimitar S-2. Both worked about the same...had to really howl to get up enough speed...I think over 11k launch. There is an 11-5 Scimitar S-2, I think...might be better. I don't understand why their pitches are so far under nominal.
A couple ideas...1) Too little break-in 2) Too small a venturi 3) Not enough head clearance 4) Maybe bad bearings.
1) These engines are not high precision stuff, so may need some extra running to come in. Still, they're better made than a Fox .35 Stunt... and real ABC! GMA liked Magnums because they were real ABC and had (generally) a reasonable "pinch" when NIB. Like a Fox, you may have to mess with them to "get all the screws in the right place".
2) Too small a venturi can be a bad thing. Less fuel = less oil and less cooling. More fuel is often much better!
3) Mike (Haverly) had one XLS .36 that had an extra head gasket in it, from the factory. He took the second gasket out, and discovered that the piston was hitting the head with only one gasket. Ooops! Won't run good that way! Probably, the factory counterbored the case too deep for the cylinder flange? Lowered the timing...hmmm.
4) I don't believe I've ever had a problem with a ball bearing failure in an engine, unless it got dirt in it and got a "hitch" in it. Shaft runs on speed engines, combat engines pounded into the sod, free flight engines pounded into the rocky prairie, etc. No shredded retainers, no problems. But I really don't trust Chinese ball bearings, either. Not on my boat trailer, not in my engines. If you have a problem with them, consult with Boca Bearing.

Steve