Now have about 4 hours on the Hushpower II muffler and it's still in one piece so time for a preliminary update

1.  It weighs 9 lbs  (4.5 less than Spintech).

2.  It is definitely louder than the Spintech but not fatally so.  Think it would sound very good on an engine with exhaust port splitters  (or a Renesis : )The GIB  (girl in backseat) says the Spintech sound level was better.

3.  The sound from HP II is more concentrated in a narrow spectrum which makes it easier for ANR headsets to cancel but  sound pressure readings in the cockpit are a good 5 - 8 db higher.

4.  The engine has more power with this muffler.  Just a guess right now but I think this may be due to more backpressure rather than less (compared to Spintech).

5.  The aero drag on Spintech was costing me more than the 3 - 4 mph that I had estimated.  It was less aero drag that attracted me to the Hushpower II to begin with. 

6.  The airplane is frighteningly fast due to 4 & 5.  Could not do an all out top speed run due to airframe concerns.  I backed off when airspeed exceeded Vne by 10 mph.

Inspection of the spintech internals (easy to see through the inlet) showed it was in perfect condition after ~ 300 hours.