OK, I just found pretty much the best feat to combo with Stalwart Planar Ally if you're a Paladin.
Namely, Despana School; it's prereqs are stuff you probably won't mind picking up anyway, and it gives the following two benefits:
1. If you and any creatures that you summoned are threatening the same creature, you are all treated as if you are flanking that creature.
2. 1/round, you can declare that your attack leaves your opponent open; if you successfully land the attack, your summoned creatures get a +2 to attack and damage on their attacks for the remainder of the round.
Hell, be a Kenku Paladin, and laugh. Really. Damn. Hard.
"So, your mount gets what kind of bonuses?"
"Oh, it just has +4 AC, +4 to Saves, +4 to hit, and my first attack each round gives it +2 to attack and damage rolls. Nothing big."