Note that with the arcane revision, Orbs gain 2d4 damage per caster level. Factoring in ER bonuses, this is 2.5d4 per basic caster level at ER5.
@Brumm: I think that build was made before the arcane rework? The loss of spell damage from the missing caster levels is fairly significant now, and is not quite compensated by the extra sneak dice (see below). Additionally, it has limitations in that not all casts trigger spell sneak, and that orbs aren't the only spells to use for damage - Ice storms, and other cloud/IGMS offensive spells remain situationally useful, like vs an enemy with Storm Avatar up. Golems are also weaker. Perhaps a revision to something like the blaster-type build suggested below would be stronger considering the spell changes?
I actually play my DSM with a similar build composition as the OP's build. Frostmage isn't there for single target damage - it's there so I can reliably blow apart groups with ice storm, and I do recognise that I am giving up single target firepower to do so. Taking the epic purist feats also lets me cast competitively with non DSM arcanes for a more generalist playstyle - while still enjoying significant daggerspell bonuses and spell sneak (runeword on weapon helps)
That said, I think there are also viable alternatives. Here are 2 examples I can think of.
1) Single target blaster: A Wizard17/Rogue3/DSM10/AT10 would maximise both CL and retain a large number of sneak dice while keeping access to purist feats. It loses HIPs and 3 sneak dice (6d12 damage) compared to the Brumm DSM, but can use teleport to break aggro and gains 9 caster levels after ER multiplication (18d4 + Epic Explosive damge per orb - you actually GAIN damage on maximised sneak orbs due to the Epic Explosive bonus). A golem with 40 CL will greatly help survivability.
2) A high AC defensive alternative: Wizard28/Rogue1/DSM10/Monk1 that would stack monk Wis to AC (6 + base stat mod) and enjoy multiple epic bonus feats at the expense of the firepower bonuses enjoyed by going Frostmage or Arcane trickster. The Wis bonus enjoyed by deep gnomes is also beneficial here, and you can spread stats more evenly.
**Sorc can be substituted for Wizard. Both have their pros and cons that I'm sure you're aware of.
***Substitute rogue with the sneak class of choice