I agree 100%.
The fundamental distinction, as you and I have discussed in the past, is between
MASS and
CORE STRENGTH. People tend to equate them, but they are entirely different. To succeed at DT you need both, but they need to be balanced. And to have movement skills and quickness as well requires an even more delicate balance.
Ron Brace is a good example of a guy who has more than adequate mass at 6'3" 330# but who lacks core strength, and tends to get pushed around. He's not a pushover because of his mass, but he's never developed the core strength to be dominant. Remember Justin Smith tossing around OLs in the NFCCG last year like they were rag dolls? Smith has much less mass than Brace, but he has phenomenal core strength, and he plays with more functional strength and power than most 3-4 DEs 20-40 lbs. heavier.
Mass IS important, too. There are players with exceptional core strength for their size, but without adequate mass they can't take the pounding and match up with behemoth OLs. Smaller players tend to get worn down by the constant wear and tear of the trenches. Justin Smith couldn't hold up playing DT for a season.
But there is also clearly a point at which additional mass doesn't add much if a player has adequate core strength, and only interferes with movement and agility. And that point is probably somewhere between 325-335 lbs., with very rare exceptions. There will be some variability based on height and frame, of course. Kwame Geathers may be another Ted Washington and play north of 350# effetively, but if so he would be the exceptional. Even huge guys like Terrance Cody have struggled at higher weight in the pros, where they are matched up against linemen with great core strength (it's easier to dominant in the college ranks based on pure size). I think there may be a little more range than you suggest, but I agree that it's very difficult to find guys over 330# who have much in the way of quickness and movement skills. Haloti Ngata dropped from 345#+ to 330# last year, and it clearly showed in his quickness and disruption:
Ravens' Haloti Ngata drops pounds, gains a step | OregonLive.com
However, some people have suggested that Ngata got worn down a bit in 2011 at the lighter weight, and that his play dropped off in the second half of the season as a result. Again, it's a delicate balance. But above 340 lbs. it's probably hard for almost any player to have optimal agility and quickness. Like you, I'm thrilled with the idea of a John Jenkins in the 325-330# range and improved quickness - though I'd prefer that he wait until after the Pats draft him in the 2nd-3rd round to make that change.