The Dexterity bonus ought to influence both movement speed and attack speed.
I'd suggest making it the full bonus for movement speed and half of the bonus for attack speed (both melee and ranged).
That way 30 Dex would give +10% movement speed and +5% attack speed. 40 Dex would be +15% movement and +7.5% attack, etc.
This suggestion should provide a little more variety to toons in the game. Who says everyone should attack or move at the same speed once they have Haste. As in real life, different toons should have different abilities.