The only real reason to go Deep Gnome instead of Sun Elf is to take advantage of the Deep Gnome's affinity for the Illusion school of magic. You'll get a race-based spell-like ability Phantasmal Killer "spell" in addition to your regular Phantasmal Killer spell.

That being the case, you'll need to go "full retard" on spell DCs. You'll also need good Spell Penetration. Unfortunately, you lose 3 points of spell penetration by not being a Sun Elf.

This is why I TRed my Deep Gnome back into a Sun Elf and just went with being a Pale Master.

You'd be surprised how much three extra "points" of Spell Penetration matters in a lot of high-end content.

The two Phantasmal Killer "spells" were fun to use in rapid succession, though.

2x necro, 2x enchantment, 2x spell pen, wiz past life, bard past life, completionist, quicken, heighten. embolden, epic spell pen, greater intelligence, scion of shadowfell, mass frog. take illusion SLAs from archmage tree for third PK. take quicken, heighten, 1st necro and 1st enchantment as a wizards feats...

Normal gnomes are better, you can't make a deep gnome look like Hitler.