Originally Posted by
Discpsycho
This is the convergence of a few issues.
First, you've got T5 in Soul Eater, which doesn't actually give you any bonus spellpower other than Hungry For Destruction. Eldritch Wave is really cantankerous, so that tree shines because of its T5 FoD SLA and capstone (which you won't be able to get). Enlightened Spirit is the only one that does, and (before the capstone) it gives 30 Light Spellpower, 12 USP, and +3d4 light spellpower.
If you're looking to maximize blasting in heroics, you'll need T5 Tainted Scholar or Englightened Spirit and 23+ points in the other, for a total of +3d4 (scaling) light damage, +4d4 pact damage, +30% spell critical damage, +30 light spellpower, +12 USP. In total that's solid, but it takes a lot of AP. Warlock trees aren't like the Sorc savant lines, which give +10 [Element] Spellpower per core.
To further maximize your blasting damage, you'll probably need to get the Flamecleansed Fury set from Sharn. I can't say by how much because I don't know your radiance / combustion gear, but if you keep Burnscar Sash, that'll bump your spellpower up further. You'd need to swap to some combination of TS / ES to maximize the bump to your Light spellpower, and because the set requires medium armor. Versus what you've got now, you'll probably be at 400 fire spellpower and 300 light, with an additional 1d4 pact damage and 3d4 light damage (roughly +10% pact dice, +33% light dice)
Ultimately, 355 / 258 isn't unbelievable, and it's certainly not bad. The non-existence of Enhancement Radiance bonuses means it'll be 77 spellpower behind Fire, and various racial trees give Fire spellpower but not Radiance.