That's why Magic TrackPad + decent gaming mouse is the best companion for your iMac.
You still get touchy thing with TrackPad, even better, more sensitive and accurate than MagicMouse .. and when you use your gaming mouse, you get comfort grip, extra buttons, much better sensitivity and accuracy of a REAL MOUSE, much better than a MagicMouse should be
The only cons is you have to sacrifice a USB port since there are a few option of bluetooth mouse out there, and even MagicMouse is not the best among them.
You may go for Razer Orochi if you really want bluetooth and gaming grade laser mouse, way way better than MagicMouse, but it's too small for me. So I chose Logitech G500