I concur with Bob L. I had close to 200 hours on my first RV-10 before selling it and had aft NACA vents. I was initially concerned about issues with static port pressure but reality didn't bear it out. Specifically, my vents were just a few inches aft of the baggage bulkhead and the top edge was slightly below the level of the static ports. Multiple GPS runs showed airspeed readings to be within 2 kts of calculated which seems to be as good as you can get regardless of having the aft NACA vents.
Airflow is very good, in fact I was a beta tester of Geoff's air controller for the very reason cited - you get too much at cruise speed! The overhead console is pressurized and aside from the mentioned air leaks, you'll get high velocity + high flow through the vents. For comfort you really need lower velocity and the controller allows you to dynamically control that. For winter flying you can close off the air almost entirely. Air leaks around switches/lights can be mostly mitigated with RTV but I was never 100% successful.
Would I do it that way again? I'm now building my second RV-10 and have aft NACA vents in the exact same location and have already purchased on OH vent controller from Geoff.
Bob