I use em they work pretty good,on 2stroke engines as well as 4st with no perceptable power loss.
if you have a muffler that has a lip on the exhaust pipe they stay on pretty good, but with no ridge to grab onto they tend to slide off even when you crank the zip ties down tightly.
I haven't done this myself but I have read that JB Weld sticks to muffler pipes,and you can build up a small lip on the end of the pipe then the deflector will hold on alot better.