Agree, a 40 yard dash doesn't say much about a defensive lineman at all, just that big guys who can run are usually pretty darned athletic
There was someone associated with the Steelers a few years back that put out some information on analyzing the explosiveness of DE's and rush-LB's. He started with the vertical jump, but think he had some other metrics he'd look at also -- sort of a size/explosiveness index that would indicate if a guy had the quick twitch to get by blockers.
It's harder to evaluate for DT's, I think, although I assume any DT with a decent vertical and shuttle for their bulk is probably pretty athletic. At the extreme end you have guys like Suh - 307 lbs, 4.44 shuttle, 35 1/2" vertical, AND 4.98 40. Wilfork's a good NT example. Like you said, fast 40, but also did a 4.5 SS, but only a 26 1/2" vertical (hard to get 340 lbs that far off the ground
), and anyways, the need there is first and foremost to anchor and control