INT does affect spell damage, but only by a minor amount. I had tested a WW and Serpentine Dragon with the same Magery/Eval, but different INT against a 0 Resists Drone with tens of thousands of Health (thanks to Kyronix spawning it in). The Serpentine Dragon had like 300 more INT, but was only doing like 20 more damage with FS to a 0 Resist target than the WW did. Once you take Resists into account, it's much lower. The only time INT makes a big difference in spell damage, is with Mind Blast, which the pet needs Magery Mastery to cast.
INT does make a decent difference in mana regeneration however. I tested a WW with 700 INT/1,500 Mana/120 Med/120 Focus against a Serpentine Dragon with 1,040 INT/1,500 Mana/120 Med/120 Focus. I let both completely mana dump, then told them to follow me and watched their Mana % with Eval Int. By the time the Serpentine Dragon reached 100% Mana, the WW was only at about 70%.
I cringe whenever i see pets that have had their max Mana pumped up a lot, but are still at only 125 INT instead of 370. Max Mana only matters for a short time, before your pet mana dumps. As soon as the pet Mana dumps, the only thing that matters is how fast it can regen the mana back. INT > Max Mana, especially in long lasting fights like bosses.