Congrats on the new acquisition!!!
Before I got into the long tele primes, I put myself in the position to choose between the Sigma 135-400 and the Tokina 80-400. I ended up with the Tokina, mostly because of size, but from what I had read, IQ was pretty much the same at the long end (which is what I was mainly concerned with). Looking back, the actual deciding factor was probably that the Tokina had recently been discontinued, and was very rapidly becoming rare to the extreme in K mount -- and we all know what happens when one can no longer get something that one perceives that he needs????
Anyway, a relatively compact zoom that reaches out to 400mm is a great companion to lenses like the Sigma 17-70, Tamron 28-75, or Pentax 28-105 for a two lens "do just about anything" kit. I think that you'll get a lot of use out of it.