this is a big assumption because there is no 1.4x or 2x HSM TC
YET
If someone develops a HSM TC for pentax why not just go for it and a 70-200 F2.8 plus 1.4x and 2x TCs.
While the purists will argue the DA300 is much better than any sigma zoom and a TC, I have excellent results with my APO 70-200 F2.8 EX and sigma's 1.4x and 2x APO DG teleconverters.
The results have been so good, I am not looking at any 300mm F4 prime to replace them.
Note this does not imply I am not looking for a 300 F2.8
edit note
one other question
why not the 50-135 and the DA300 plus a TC . Unless you feel you need 200mm at F2.8 this would be lighter than the other combos.