Ok, I asume you pikc up the Fiendish Ancestory whcih gives +2 to CL when summoning fiendish or evil outsiders whit summon spells right? Spell Focus; Conjuration and Augment Summoning sound nice combo to such role
. Extend spell then of course increase the duration of the summoning spells. Also good Conjuration spells are Orb of
spells from Complete Arcana and from PHB Melf's Acid Arrow is nice one too.
But I point out that Warlock haves invocation called Dead Walk, which means animated dead spell which cna raise dead fight for you, even whitout costly spell component, for 1 min per level. Also I can allow custom made Warlock invocation whcih allows summon a creature like whit Summon Monster III spell but total HD what you can summon whit spell would be egual to twice your warlock level and you can summon only fiendish creatures or creatures of your alignment only.
But summoning sorc is good too :). Nothing wrong whit that