From my own personal point of view, I really don't see this as an extreme deviation from the fluff, and the use of human cultists as the lineman position certainly fits well enough, especially when you consider the aspect of perfecting the killing art, rather than just brute force. Nor does the lack of mutations overly concern me. They could possibly have added them as a double on the Bloodletters, but I wouldn't have given them more than that without it being both against fluff and balance - by and large the creatures of Khorne don't really favour mutations, and most of those available wouldn't really fit on that basis. Bloodletters are probably the only player I would consider notable weak compared to fluff - but making them stronger would have required a significant decision on balance. Make them Strength 4 and I'd have upped the price to around 110-120k each. The Bloodthirster was never going to be easily balanced, I doubt Cyanide were in a position to dictate new rules such as the instability and other attempts to moderate the far more powerful versions of the Bloodthirster from earlier editions, so toning him down was the least painful method of including him - its that or face a 250k big guy.