Speed in and of itself isn't killing anything.
It's speed with the lack of some kind of acceleration factor.
Even a rudimentary "acceleration = how much delay before you start running" that was in FBPro would help make a difference. Then add in the AGI factor of losing speed while changing directions and then how long you were at that speed before going to top speed again (ACC) - it could really change some dynamics.
Also, the full speed scale COULD work, but it would be slicing time into really thin slices, like .01 of a second.
To answer your question about Bolt - a game should use his top speed and use acceleration to make his overall average time. For example, if his average speed was 20 mph and top speed 27 mph, that's a sign of slower acceleration, so that rating should drop. I would NOT use 23 MPH as Bolt's top speed. It is NOT his top speed, but his overall speed over the full distance - that's where ACC comes in.
Basically:
SPD = player's top speed. Don't tinker with this - if he's blazing OMGLOLWUT fast, then so be it.
ACC = time it takes to get to top speed. Use the delay before running if nothing else. If someone could cover 40 yds in 3.6 second at top speed, but does it in 4.3 - then there's the delay.
AGI = the same role as ACC, except it's used in situations where a change of direction is performed. A HB with high AGI would maintain his speed easily after a juke or be a good "one cut and go" type of runner. A WR with high AGI would make a good route runner for complex/multiple move routes (slant and go, out and up, post-corner)
So:
100 SPD, 0 ACC, 100 AGI - guy who changes directions without losing much of any speed, but takes a LONG time to get up to that speed. These are guys that might disappoint in the 40. You can see their speed, but they don't look like they "run hard" or have "good technique".
100 SPD, 100 ACC, 0 AGI - guy who's freaking fast and gets to top speed in a couple steps (very low delay), but if he has to change directions - he struggles mightily, making him just a good straight-line runner. This type of player will show well in the 40 - so a top 40 time would be a sign of SPD + ACC being high. AGI would be unknown since there's no changing direction in the 40.
0 SPD, 100 ACC, 100 AGI - slow as dirt, but at least gets up to his speed fast, and if he has to change directions, he does so easily and maintaining his speed. These types guys might have a decent 40 times and could be useful on the field, but might slip because they "look slow". These are the "sneaky fast" types (not with 0 SPD, of course, but lower SPD but high ACC and AGI)
Stuff like that would really change dynamics, imo.
And was done back in '98 heh