Well, here's what I've got in the way of triggers right now...
Event: Generic Unit - Unit uses Tome of Speed [where ToS is a technically useless item]
Action: Unit - Add [the speeding up ability associated with Boots of Speed, which has since been set to also be a hero ability] to (triggering unit)
The problem with that set up is that you can only get one speed upgrade. I mean, I could have a series of increasing bonuses and create a long, long chain of triggers, the first of which would give the hero a 60 movement bonus the first time the hero buys the tome. Then, I'd have to make a trigger that, when a second tome is purchased, destroys the first ability and replaces it with a second bonus of 120 and so on. That wouldn't be too awful if it were only a one player map, but it's a 12 man arena. I'd have to do multiple triggers for every player and that sucks. I was hoping there'd be an easier way...