this "spell haste has bad mana efficiency" stuff doesn't work out in reality. if you cast 30 frostbolts in 75 seconds, you do X damage total. if you cast 30 frostbolts with spell haste in 70 seconds, you still do X damage total. X/75 < X/70, therefore the DPS is higher without losing any total damage output.

if you're going OOM, in other words, you would have gone OOM anyway, but your DPS during your damage-time would have been less. if you need to have mana during certain parts of the fight and DPS is a non-issue, throttle your casting.

Nick,
your comment only makes sense if you are the only DPS on the target. In your example you would increase DPS by 5/75 (roughly 6,7%) at the same time spending 6,7% more mana/s. If other DPS on the target don't increase their DPS in the same way the fight will take you subjectively "longer" than before, since you have to deal a larger portion of the mobs health than before to kill it, thus increasing your risk of going OOM.
To sum it up, spell haste increases DPS and MPS by excactly the same amount.