You do realize that you can only use Uncanny forethought on a number of spell slots = your intelligence modifier, right? You realize that, even if you started with a 20 intelligence (grey elf) at first level, that you have more spell slots than you can contribute to the UF pool starting at level 1 (if you count 0's) and level 3 (2nd level slots don't help if all you have is 1st levels with spell mastery)? That, unless you take spell mastery at level 3 or 5 or 6, NONE of your spells higher than level 1 will be standard actions when utilizing UF (can't UF nerveskitter if you don't spell mastery it, and even then, you just spent one of your 5 slots for UF on +5 initiative)
How about this change then:
Add a number of spell slots to your UF pool equal to your wisdom modifier. Can only take it once.
ONLY int mod spells per day can be cast spontaneously? Are you sure you know what the word "only" means? Because it usually refers to amounts of stuff that are considered rather too much on the small side.
4+ spontaneous spells per day is
huge for wizards (that's about 6+ at 8th level plus, and can easily reach 10+ at the highest levels even for non-Grey Elves), even at the cost of two feats. In a recent PF campaign, I was stoked to get
one spontaneous spell per day via Bonded Object. That option saved my life, or at the very least made things a
lot easier, on multiple occasions. Having that Knock or Comprehend Languages or Levitate or Daylight or Ropetrick at your fingertips at all times makes for easy, sweet adventuring - not to mention higher level options. You don't even have to take these with Spell Mastery, since casting them as a full-round action is usually sufficient. And for 'downtime spells' like Identify, Legend Lore, Major Creation, Symbol of X, or Planar Binding, you can now use them as a full-round action whenever you like, even in the midst of a dungeon crawl.
Now Nerveskitter isn't really a good example, because casting it as a standard sucks hardcore, so you should memorize it rather than UFing it. But suppose it works; then your wording "even then, you just spent one of your 5 slots for UF on +5 initiative" just begs for the retort "you just spent one of your feats to get a better version of Improved Initiative, which is not a bad feat in itself,
and 4 spontaneous spells per day".
Sure, with UF a lot depends on your Mastered spells (i.e. the timing of your taking the Spell Mastery feat). But the option of spontaneously casting
anything in your spellbook as a full-round action is still extremely good. Not to mention that you can take Spell Mastery multiple times, or even retrain it for Spell Mastery of different spells via feat retraining.