Sounds like a real freak. I would have had a hard time not putting him at WR though. 99 speed means if he can get off the jam at the line he can get open deep all the time, and speed would come in to play more at that than anywhere else. I understand the roster situation though. If you have an RB with low 90's speed, he will generally be able to do all the things a guy with 99 speed can, since low 90's will still beat most of the defense to the edge on outside plays, and its rare that an RB gets into a race running up the middle. And crazy as it sounds, running up the middle with a guy with extreme speed can hurt you because you'll run right into tackers, whereas a slightly slower guy will give you more time to read the blocks and adjust. Catch ratings also really only matter when getting jump balls or when the receiver is taking a hit right after the ball getting there.
I had a WR who was a 99 speed. He easily won returner of the year rookie year, and was near impossible to cover on the outside.