X-heads vs J-heads: what's the difference??

I don't mean any offense to anyone when I say this, but as much information as Jim Laroy has posted on here, I think the old "big valves for high RPM" myth is just that. A myth. Yes, you can argue that most of what Jim posts is about big block stuff, but IMO it will be the same across the board. The bigger valves, along with the correct corresponding other mods to take advantage of them do make more power through the entire RPM range. He's proven it time and again. An engine is just an air pump. It doesn't know what size air pump it is, so argue it all you care to. The "Oh but that's a big block" argument doesn't hold water, IMO. Bigger valves = more power.