[Rupert]

I would certainly apply a speed penalty to any ranged attacks on a speedster who used their extra moves to zip about, and probably to any melee attacks via 'wait' actions. If they end their turn in reach of someone unless they've been running around them in circles I'd be inclined to not give them the benefit of this - bad planning should have consequences (or good planning on the part of the opponents).

Thus if an otherwise average character had ATR3, for four actions per turn, they could Move (run 5 yards), Move (sprint 6 yards), Move (sprint 6 yards), and Move (sprint 6 yards), for a total movement of 23 yards and -6 to ranged attacks against them (aside from those of someone they were running right at or away from if they didn't think to put a few turns in there). If they turn that last Move into an All-Out Defence (Increased Dodge) they move 19 yards in total (-5 to hit) and get +2 to Dodge.

I think that fairly represents using the extra actions to not get hit.

If one wants to reflect a more reactive increase in speed, Enhanced Defences is clearly the way to go (and I'd probably expect all to be bought at once), and I'd allow ATR as a prerequisite for it. I'd most likely use the recommended cap of three levels, and I'd certainly not allow more than +1 Enhanced Defences per level of ATR (Looking up the total number of actions on the size table for a cap would be appealing). Extra bonuses to defences, especially Dodge is very powerful, and they should be approached with caution.