Hm, I would be surprised if the Tamron zoom would actually outperform the Samyang 85mm. Keep in mind that the Samyang also allows much faster apertures, which makes a shallower DoF and smoother bokeh. I don't have the Samyang 85mm, but I heard its a great lens, apart from the focusing. Its MF only, the distance scales might not be accurate, and on some copies the AF ring is very very hard to turn. But its cheap and allows some very creative photos. The 100mm macro might be a good choice and it even allows true 1:1 macro and WR.
What about the
DA* 55mm F1.4? Pretty much made for portraits, supposedly renders skin tones very well, allows shallow DoF, and is a very comfortable focal length on APS-C.
Or the
FA 77mm f1.8? A legendary Pentax lens. I think these two are the current main "Pentax portrait lenses"
But on the other hand, if you are happy with the performance you get from a lens you already own.. seems pointless to buy a lens that won't have a dramatic difference in IQ.