Surely you can test the impact of this now to see exactly how you'll be effected.
Have only one shaman drop mana spring when you do your normal instance runs.

At least this way you will know one way or the other whether your team is still viable as is or if you need to look at alternatives
to improve your mana pool, enchants, mana pots etc.

You might just have to tweak casting rotations. So you dont go for max dps - just balance the dps needed to kill the boss with mana usage.