Well, Summoner's strengths:
-Eidolon makes for a very good pouncer, and in it's own right, qualifies the class for mid range T4 already, but no more.
-Summon Monster SLA is plentiful, fast, long lasting and flexible. This would give it low T3 status on it's own, but the summons aren't that spectacular in combat without the buff spells.
-The spells supply much needed BFC, buffs and debuffs to let the summons excel, and carry out essentials. However, at 6th level progression with a limited know-list, it's not much of a caster, and offensive spells will have pretty crappy DCs.
What makes it T2 are specific spells mostly, and you could bring it down to T3 by trimming these: Charm/Dominate Monster, Contact Other Plane, Major/Minor Creation(in groups where abusing creation is possible), Planar Binding, Create Demiplane(though you lack the Greater version for time and morphic traits), and Gate.