If memory serves from the guide, Malconvoker 5 or 6 are great break-points for the class. You don't really get anything awesome after that, so feel free to jump ship at that point. Don't know of many other summoner PrC's right off my head, although heading into Comtemplative for another domain couldn't hurt.
Also, you will need to rework your feats some, as both Metamagic School Focus (conj) and Augment Summoning require School Focus (conj) to take. You can cheat on AS with the summoner domain, but not with MMSF. Malconvoker doesn't actually give you twin spell, just something that approximates it, so you can't really take DMM Twin at lvl 9. Other than that, I would recommend either Extend Spell, along with picking up a Ring of Mighty Summons, or an Extra Turning or two. Looks like you're on the right track though.