If you're an MMO player, you should just get a gaming keypad (Razer Nostromo, Razer Orbweaver, or Logitech G13). It's pretty much all you will ever need to play MMOs and takes away the need for a mouse with tons of buttons. Your thumb isn't that dexterous when it comes to pressing multiple buttons quickly and I find that the thumbpad on the gaming keypads gives slightly better reaction time than WASD'ing.
I have mastered my Naga 2012, and can effectively cast all my abilities in rhythm with the global cooldown, while maintaining pretty much 100% "needed" accuracy (no loss of LOS, always tracking the target.) I also play the stealth / rogue class in my MMO of choice and moving around while executing abilities isn't a problem either. I am by no stretch special, or gifted in hand-eye coordination. It took me about 2 to 4 weeks to acclimate to adequate skill level, and a couple months for it to fully sink in to where I could execute any of my slots without batting an eye.
WASD is not slower or quicker than the Orbweaver's thumbpad. There's no physical way for it to be "more responsive"
In conclusion, I think what you're effectively doing by going from Naga to Orbweaver, is taking some work away from your right thumb, and offloading that onto your left thumb and left-hand fingers, which just equates to extra work, IMO.