A few things:
1. I think Davis was very lucky to be the right guy, at the right time. He had one hell 3-4 year run in the league. It was also over 25 years ago. It's pretty dumb to think that just because Terrell Davis was a needle in the haystack a lifetime ago that suddenly a guy that is slow on tape and slow in the 40 will be the next Davis.
2. Davis was aided by that Denver team being just a great team - the OL, the scheme, and having a HOF QB. Again, right guy at the right place and time.
3. I do think some guys stand out where they are strong in a way that they do not lose a step when the pads come on. I can't remember now, but I would not be shocked to see Davis fall into that category. I always felt Urlacher was also like that - Urlacher was never considered 'slow', but his 40 time was 4.57 and yet you would see him run down guys that had much faster 40's. I do believe at least some of that was that those smaller, faster guys slowed a bit when the pads came on and Urlacher just didn't slow down with pads on. Again, I think Davis was probably similar.
4. IMO, overall speed of the game is faster now then it was 25-30 years ago. Hard to say how a Terrell Davis would be in today's game when modern linebackers now run 4.5 and sub 4.5 40's consistently and with the strength to keep the speed with pads on.