17x6 or 16x8 are what many/most are using, will both hold the engine to something in the 8500+- range. It's not unusual for tuning to change cowl on vs. cowl off. Have you tried changing the tuning to accommodate flying with the cowl on? Could be a heat issue, or air pressure changes due to ram air effect? I've never had to do the remote vent thing, and I've never witnessed that as a cure for any issues I've witnessed first hand. Others have claimed/will claim differently.

A pic of your plane's cowl while installed on the plane might help identify potential cooling issues, or confirm there should be none?