Recently ran some theoretical cooling numbers for the RV8, and I do not think the exit area is too small. I think Van sized it just right for an average installed setup...and frankly, I think most of the guys with high temps have baffles in need of work.
The single most important variable (small change equals big difference) is heat transfer efficiency ratio, cooling air temperature change /(CHT - ambient temperature). Put another way, your goal is to raise air temperature as much as possible. Consider a 380 CHT on a 60 degree day. Raising outlet temperature by 20F (80 vs 100) is a bump from .25 to .3125. Doesn't sound like a big deal, but......
The RV8's exit area is between 0.4 and 0.45 sq ft, depending on how much area you squander on exhaust pipes. At .25 ER, the sample calcs say you should be able to cool 174 to 183 hp with 2.8 and 2.9 lbs/s at 100 mph. At .3125 ER, you should be able to cool 214 to 225 hp at the same speed. Or you could cut exit area to 0.3, still cool 180 hp, and lose some drag; required air is now about 2.35 lbs/s.
So how? First, make sure your baffles really fit, hugging the cylinders and heads at all points so every bit of air
passes between fins with maximum surface contact for the maximum possible distance. A standard set of Van's kit baffles don't fit right from the box. Many builders are happy when they get them trimmed to fit on the engine, and stop working.
Below is the stock Lycoming intercylinder "baffle" supplied on my 390. It doesn't even touch the fins as delivered. Somebody placed a strip of orange RTV across the end of the baffle, but it doesn't do any good; a lot of the air will leak out the side gaps (small arrows). For sure this air never makes it to the vicinity of the fins inboard of the spark plug, where it should be exiting the baffle (big arrow).
Another view of the same spot, and the other wing of the same baffle. See the gaps?
Here we have a Vans baffle wrap on the left rear cylinder. Lycoming puts a rubber comb on the fins so they won't vibrate. The baffle wrap rests against the rubber comb, rather than the tips of the fins:
However, look what happens to the baffle wrap. Again you get a big 'ole gap along the sides. You figure the air is continuing straight ahead between the fins, over the rubber comb, and exiting the end of the wrap, or going out the sides?
BTW, this is after the wrap was bent to conform with the sharp corner on the cylinder head fins (big arrow). We've all seen lots of these which were simply pulled around the head with no forming, and the gaps are huge.
Air not in in contact with a fin surface doesn't pick up any heat. The really sad thing is that the above baffle wrap is the one near the exhaust port.....the hottest part of the cylinder head. Remember delta-t? This is the area with the most potential for heat transfer.
Ok, so much for fixing stock baffles to work better. Now consider some of the work done by the canard guys, the sho'nuff fanatics of drag reduction. The fast guys don't use baffles like ours. Ours wrap only 60 or 70 degrees of cylinder on each side. Some of theirs wrap 120-130 degrees.....and are often formed from glass and RTV so as to not leak anywhere along that wrap.