Summoner is incredibly powerful. I have designed smacks dealing thousands of damage at level 12 with that class. If your PC manages to get that class and combine it with the summoning PrD he will be unstoppable.
Btw - the best summoners will always be clerics. They can take alignment domains increase the caster level of the summon spells that are of that type netting +2 caster level (and with the summoning domain double their caster level), and they have access to spells such as bless, prayer, recitation, and Righteous Wrath of the Faithful (I think that is the name of it - the one where all of your devout allies gain an extra partial action - stackable with haste!!! - and bonuses to hit and damage).
Picture a 5th level cleric with law and good domains who progresses in summoner class until he can get to the contemplative so at level 11 he will be Clr 5/Summoner5/Contemplative 1
His effective caster level when casting summon spells (including the planar ally spells) is 26. Use all the 4th, 5th, and 6th level spells slots (as well as the 7th granted by the summoner class) to summon lantern archons (the 7th would be an empowered 5th level). That's ~ 13 spells (assuming high Wis) each taking a full round which yields ~24 lantern archons. Better yet, have your sorcerer cohort cast haste on you, him and every one of your cohorts (=26 hastes, will at least be able to cast 19 automatically, lets assume he has a few scrolls to finish the job). So you are both at 13 rounds of actions. Finally you cast bless and recitation. Your archons meanwhile were all aiding themselves. You took the augment summoning feat (bringing your feat total to 3 so far) so each of your little archons has additional hp, and att/dmg bonus. All at once your archons teleport to your enemy (w/o) error each getting 2 shots in (from the hastes) netting 26d6+104 total damage possible with touch attacks at +7 to hit. lets assume they hit 75% of the time and do average damage. That means 146 damage. That was just the surprise round. Let's go ahead and make this a smack. Lantern archons have II so let's say that roughly 50% of themwill win initiative next round (assume enemy also has some initiative bonus). Now those that win initiative get 3 attacks adding in an additional 110 damage That certainly gets you into smackdown territory. And this is only at level 11. Moreover, and more importantly, this damage is done in 7.5 dmg increments that ignores damage resistance. So that's ~ 34 7.5 dmg dealings to distribute however the archons see fit that cannot be reduced by any means.
Add in Divine Disciple to be able to telepathically communicate with any summoned creature and the other 5 levels of summoner and this character is literally unstoppable. He just sits in his fortress summoning his little lantern pets to teleport thousands of miles if need be to dispatch his foes.
