Platehelm of Irate Revenge is a decent platform to build on - to do much better you're looking at heroic drops, Naxx-10, or championing your way to Exalted with Argent Crusade.
But yeah, one piece of titansteel or saronite is an extravagence that you can live with, but multiple pieces just gut your mana pool - and fights no longer last long enough for mp5 to make up the slack.
Your spellpower is fine, my advice would be look for diversification - target upgrades that give you +sp, +int, /and/ either crit or (failing that) haste. Get your mana pool approaching 20k unbuffed, and you can mix in a lot more (glyphed - 20y radius after the patch!) holy lights, and that will increase your throughput more than the same itemization worth of +sp, and ease mana worries to boot.
Edited, Jan 7th 2009 5:31pm by ElMuneco