Mages' mana gems are receiving another change in the next patch. The range of the mana returned will be narrowed, giving us a more consistent boost. On live realms, the best gem gives 1136-1364 mana. The PTR previously showed a change: it was increased 1800-3000. Now it has been changed again to 2340-2460.
I think this is a buff. Consistency is almost always a good thing in general. Right now, when raiding, we mages (and other mana-using classes) will wait until our mana pool is 3000 away from the maximum before using a [Super Mana Potion]. That way, even if the pot gives its maximum return of 3000 mana, we receive the full benefit.
With the upcoming changes, we can use these gems when we are 2500 (2460 to be exact) away from maximum mana. Couple that with the other mana gem change (each gem having three charges), we will be potting earlier in boss fights, but I think we'll be potting about the same number of times.
Overall, I think we are only getting consistency, which will serve to make us better raiders.