From what I understand, spell damage is a factor of the % visible in guild, your level, and some specific formula for each spell. So since wil does not factor into spell % in guild for mage attack spells, wil should not factor into damage for those spells, such as spray/fball/bolt, only int, wis, and per.
As for the level vs. %, they are both a factor. Let's take a level 25 mage with 105% is sleep for example. His spell level comes out to 25 * 105% = 2625. A mage with 101% sleep needs then to be 25.99 (real level... so actually about level 35) in order to have an equal spell level. However, I think there is also some factor of target's level vs your level that will affect success, so the level 35 mage probably will succeed better even with the same spell level than the level 25 mage.
If I'm wrong on this someone correct me please, cuz I don't really play mage much!
And 12 wil mage vs 15 wil, 101% sleep vs 105%. Would it really make that big of a difference if you levelled up to 50?