I went to the annual RV fly-in at Weatherford, OK today and saw an interesting RV-6 with some cool aero modifications. The owner said he has it going 209 mph now on only 150 hp. He has a Paul Lipse prop but also installed a plenum with the intake area cut down 2/3rds from the stock cowling inlets and also added an exhaust area fairing as seen at the attached link.  He said that just be adding this exhaust fairing alone added 9 mph to his top end...

 

Doug Lomheim

RV-9A / 13B