This thread has definitely inspired me to add a Culexus (and maybe a few other assassins as well) to my army, and I'd much prefer a converted model myself.
However, that statline makes it almost impossible for me to consider using an actual Tau as the base - I don't care how good Tau battlesuit technology is, it isn't granting a Tau a WS and I nearly double that of a Space Marine, especially given that Farsight (a commander who has lived significantly longer than any other Tau and who wields a melee weapon adroitly into combat) is below the Assassin-level WS (For now... I plan on representing my Horus proxy from Forgeworld's Horus Heresy as an older Farsight finally fully attuned to a few artifacts... Yes, yes, warptau).
...Well, I might make an exception for Tau in larger battlesuits. Or, say, the "impossible" results of an captured but eventually freed Earth Caste engineer "eksparamint'd on" by a Mad Doc... Yeah, my conversion ideas tend to be a bit weird and silly
The only exception I can imagine would be a chaos-infused Tau. Kais was mentioned, and that might work, especially given the relatively unique level of chaos possession, especially given that possession was Khorne, notoriously anti-psyker...
An additional ally race would be intriguing, or even a perversion of an existing one... Nicassar would be one possibility, though that would be a fairly extreme level of modification for the Tau to go through... wouldn't it?
But given the theme of my own conversions, it seems much more likely I'll be converting a Kroot. The source of their anti-psyker elements seems a bit easier to swallow (hehe), given the Tau have at least a few planets of humans, and my personal background has the Tau experimentally "guiding" a small contingent of Kroot "volunteers" through perfectly-controlled dietary restrictions.
My Tau Empire are done being naive, and through observance in battle or the fearful whispers of their more psychically attuned allies, will undoubtedly have learned about the existence of nulls and their effect on psykers. Given enough time, delicate enough instruments/allies to find those nearer the null spectrum, and sufficiently unscrupulous experimental methods, a sufficient breeding population of nulls could probably be established within 50-100 years. Given the unscrupulous nature of the experimentation, this activity would probably be off the books, and known only to high level Ethereals and a few Earth Caste sworn to secrecy (as well as the droids that help them).
How long has it been since Damocles, again?
Certainly not enough in that time frame to build an Assassinorum, but likely enough to build a reasonably sized Kroot Null (Knull?) population, from which perhaps 1 or 2 of the best and the brightest might be trained to the impossible levels that the Imperium Assassins represent.
...Dangit, now I'm excited. I have finals to study for, not modelling plans!