da* 16-50 f2.8 artificially restricted to f2.8?

I understand the tstop and fstop thing.. and one of the reasons cine lens are expensive as unlike my sigma that varies tstop as i zoom (at f2.8) they are built to a higher standard to stop this.

Im guessing my lens isn't properly calebrated as I asked one of my online Pentax friends and while his lens its broken now he can remember it having circular oof highlights wide open where mine doesn't as the blades are slightly shut.

Now since i got the lens second hand.. warenty won't cover it and not sure if sending it back would work either.. I payed £412 for it so a good price.

Do I just live with it as is? Is it easy to recelebrate? Or do i remove the stopper or even try swap it for a smaller one?

I'd like to see some more images of the lens wide open at 50mm to see if its just my lens and I will do some tests later with continous lights and a tripod with a few lens at f2.8