I would assume that Huton and Greased Lightning would NOT proc the extra regen, obviously. Of course, it would all depend on how SE coded them, which we have no idea about. And while it is true that in terms of raw TP restored would be considerable, it should also be noted that this same amount of TP that would be extra amount consumed as a result of going skill speed as opposed to crit or det.
And the fix isn't so much intended as a cure-all for SMNs, merely a bone to throw their way. The actual intention of this fix is to make spell and skill speed not a dead stat to anybody not BLM or WHM (and the latter is hotly contested). If you wanted to fix the SMN mana problem, you'd do:
A) Make enhanced pet actions give you back 2% of your mana back each time your pet DAMAGES an enemy. SCH cannot summon a pet that can take advantage of this trait, and it makes the SMN reliant on keeping his pet ON the target for sustain and damage, which is, you know, the POINT of a summoner. The amount of mana you get back should be enough to make SMN last indefinately, allowing it to compete with BLM.
B) Give a large amount of PIE on the SMN crystal or add some sort of INT to PIE ratio for the job.
C) Some other simple idea that doesn't touch SCH in that massive thread that has beat this topic to death (and rightfully so).
Although, since you brought it up, let me do some math. SMN has ~3000 MP at level 50, which essentially makes that extra tick per minute restore like 60 MP. Assuming that the base rate of net MP consumption during a standard rotation (w.o Ruin II) is 867 MP per Minute or 28% of total MP per minute spent, this extra MP from 126 spell speed would basically increase their time to OOM from ~3.5 minutes to ~3.75 minutes when going full tilt. You are right in that it's not much of a difference at all, but it is still something. I should make the point that the extra 15 seconds would be approximately what SMNs lose by using spell speed gear (although the main reason why they hate the stat is due to DoTs not scaling with it, of course) -- which, I guess, makes the 2.4 gear simply a grain of salt in a gaping wound.
The funny thing is that small differences in MP and TP regen often make substantial differences in time to OOM, especially when the net rate of consumption is small. Ask any healer that loses a bit of PIE when changing gear or any NIN after patch 2.4 -- the % change is quite small, but players will often notice a difference. I'm actually been meaning to do a more in-depth proof of this in the healing forums.
EDIT: Made an error in math before. Corrected.