From my end I can tell you I noticed more difference in the sound than the power. I was running a worked over 318 at the time; small valve J heads, LD340. I don't remember the cam specs, but just to give you an idea it wouldn't support power brakes.
With the Doug's it seemed to feel like I was getting a bit more power throughout the range, but when you spend $850 on something you have to watch for psychology creeping in and making you see things that aren't there. The Doug's are coated so I'm sure temps are down under the hood, but I didn't actually notice that either.
Bottom line, those Headman shorties perform way better than conventional wisdom would suggest they should.
