Cub bloke here..
My L4 cruises at an honest 63kts with the doors closed. With the doors open two things happen: Firstly, the ASI under reads due to the change in static pressure in the cockpit. Secondly, all that extra drag slows you down anyway.. so we're looking at about 65mph indicated rather that the usual 75 - either way that's fast enough to get a goodly breeze through the open doors. I know cuz I did it yesterday, today and I probably do it tommorrow too!!
Kingy