RE: Does exhaust diverter decrease power??
I had a deflecter on my O.S 40LA powering a trainer and a guy at my club told the same thing (that it takes away RPM's)
I took the deflecter off on my next flight and noticed the difference.
Even though you get 'junk' all over the model it is worth it.
I bevieve that without a deflector, the engine has a better idile too