Ventilation

I am considering installing 2 rectangular vents (RV-10 kit) on each side of the fuselage just behind of the firewall to increase fresh air coming into the cabin during the summer. Any recommendations?

Do you mean in lieu of the standard Vans vents that are installed just behind the firewall?

I've flown in a couple RV-7s with the standard vents and they seemed quite adequate. Most complain that the stock vent controls do not seal well in colder weather. The Steinair metal vents fix that, for a price.

In addition to the vents (eye ball vents)

I've flown an RV-6 with standard vents in the middle of a Georgia summer; I imagine a -7 is no different. You get plenty of air coming in once you get some speed (or are you trying to ventilate your feet, too?). Then again, when that air's 90?F or more and loaded with humidity, there's not a whole lot it's going to do to cool you off, no matter how much of it is coming in. Your only solution then is to climb as fast as you can into cooler air.

I flew my -6A for a few years and ended up adding one rectanglular vent on the pilots side about 12 inches aft of the instrument panel. I keep it open most summer days. This is in addition to the standard eyeball vents.