Assuming a Specialize Alteration skill of 255, a 15% mod should save you 1.9% of the _base_ cost of the spell. This means the more mana preservation and mana regen you have, the greater the relative benefit you'll see.
Some calculations as a druid attempting to maximize the length of time that I can heal on an "endurance" fight: (Note that I have not experimentally verified these numbers; these are based on my understanding of the calculations behind mana preservation.)
As a druid I use Ancient Chlorobon as my primary heal. I have SCM3 (10%) and a 25% mana preservation focus for that spell (average of 13%). Specialize Alteration 255 = 12.75%; with Specialize Alteration +15%, that becomes Specialize Alteration 293 = 14.65%. Assume that I'm using a Benefit Awareness potion for 5% mana preservation on top of that.
Suppose I have 178 mana/tick regen normally, and that I have up for 18 ticks out of every 100, which provides an extra 18% mana preservation (in place of the 5% from Benefit Awareness) and 50 mana/tick regen. With Mastery of the Past fizzles do not apply.
(Edit: I forgot to factor in Gift of Mana. This can be approximated by multiplying the final mana cost by 0.9.)
Suppose I cast Ancient Chlorobon once per tick. The base cost of Ancient Chlorobon is 723 mana.
* Without Specialize Alteration +15%, without Form of the White Wolf: Average mana preservation is 40.75%, for a mana cost of 428.4 (385.5 after GoM; net loss of 207.5 mana/tick).
* Without Specialize Alteration +15%, with Form of the White Wolf: Average mana preservation is 53.75%, for a mana cost of 334.4 (300.9 after GoM; net loss of 72.9 mana/tick).
Weighted average: -183.3 mana/tick.
* With Specialize Alteration +15%, without Form of the White Wolf: Average mana preservation is 42.65%, for a mana cost of 414.6 (373.2 after GoM; net loss of 195.2 mana/tick).
* With Specialize Alteration +15%, with Form of the White Wolf: Average mana preservation is 55.65%, for a mana cost of 320.7 (288.6 after GoM; net loss of 60.6 mana/tick).
Weighted average: -171.0 mana/tick.
In other words, under optimal conditions using Ancient Chlorobon once per tick, I can last 7.2% longer just by adding a Specialize Alteration +15% mod.
"Real world" results obviously won't be as impressive, but it's still nothing to sneeze at.