Quote: I like my DA15, but it can't match my Samyang 14mm. And it's why there's a DA14.
If memory serves me well, the DA 14 does't match the DA 15
If that's your argument, you don't have an argument.
DA 14
DA 15
The DA 15 is always better in the centre, and for landscape, ƒ5.6 to ƒ11, it's better centre and edge, being excellent at the edge at ƒ8,. The DA 14 is never excellent at the edge, anywhere in it's range.
I could go on, my DA 18-135 kicks the DA* 16-50s butt from anywhere but about 18mm if memory serves me well.
FA 50 macro definitely holds up compared to the DA* 55 1.4 or FA 50 1.4 so one has to ask, where are the examples that prove this point?
With Pentax lenses to date , my guess is the exact opposite is true, these are three random lenses. In three tries i didn't find one example to support your notion.
It might be possible that theoretically it's true, but in that case clearly, manufacturers have trouble producing the lens tolerances that would make it true practically.
But thinking of the compound lens diagrams I had to analyze 50 years ago when I took optical physics in a lens design course, I am really suspiscious. Practically, the sharpest part of the lens is the smallest aperture, until diffraction sets in. Most lenses on APS-c and FF are sharpest at ƒ5.6 although the very best may be sharpest at ƒ2.8 or ƒ4. An ƒ4 lens should be fine for producing an ƒ5.6 image.
Only the 31 and DA 70 qualify in the "really good lens that is sharper at ƒ4 than ƒ5.6 " category among Pentax lenses, maybe other manufacturers have more and there are examples from other manufacturers, that don't really apply to Pentax glass.