Hello, I have a DA17-70 that I have sent for repair in Canada almost a year ago, for SDM problems

and also sharpness problems. It came back with SDM repaired

, but still with the same sharpness problems. I live in Africa so I took the lens anyway because I needed it (I was on vacation in Canada when the lens came back). I have a few months left on the warranty, and I need your advise before I pay big bucks to send it for repair...
My feeling (confirmed by MTF tests) was that the lens was bad between f4 and F8 (inclusively, which is weird) for focal length > 50mm. I did a side by side comparison of DA17-70 and DA55-300, both @ 70mm (so I am comparing two "consumer grade" lenses). DA55-300 is much much better between f4 and f8. The DA17-70 starts to be good at f11 (what lens is not at f11??). I was wondering: is it suposed to be like that, i.e. do I risk to have Pentax repair say my DA17-70 is fine (and therefore loose money and time)? Note that at in the range 17~30mm the lens is very sharp at most apertures (a bit soft at f4 but still fine).
Thanks in advance for you help.
Grégoire.
PS Attached is a 100% center crop.