The main problem with sneak atk DPS is it doesn't scale well in epics because it doesn't benefit from all the lovely crit bonuses in EDs. [Same problem with glancing blows.] So you can build sneak atks as a
supplement to your DPS, but you shouldn't plan on
supplanting your regular (crit-based) DPS by relying on SAs, IMHO.
The reason I proposed a 12/5/3 split is I think T5 Swashie abilities complement your build concept better than the other T5s. In particular, I consider T5 Harper pretty weaksauce and there are enough options for
Imp Deception that I don't consider it a major incentive for Harper.