Lars,
Hive beat the "mounting" problem, so this MB.com test was only a 50% washout.
But insulation is "dead air", and why couldn't a second slightly larger sleeve of copper, (with two dollops of solder in the gap between the tubes, to prevent rattling), be added to Hive's idea?
If soldering $10 of copper, slipped over the exhaust before permattachment, doesn't knock sound down 35%, I'll owe you a gallon of gas...
Okay, maybe 25%, gas is going higher than copper.