I was thinking about this the other day as well...for all my toons.

For a multiboxer I think it will difficult to get max DPS with any mage spec. You constantly have to watch for some proc to go off. There may be a way to work the macros to maximize taking advantage of procs, hotstreak for example (I know someone on this board was trying to do this), but I haven't worked that out yet myself. However, frost might actually work out to be better due to the water elemental contributing a lot to the DPS.