I have the VL58mm f1.4 SL II, but I never used the K55mm. On the other hand, I have a FA50mm f1.7 and A50mm f1.2, and I am happy to share my thoughts on the VL58mm f1.4.
The VL58mm is very sturdy and perfect for harsh outdoor conditions. I shoot outdoor and this is critical to me. The IQ is superb. While I tend to prefer usually the IQ of the FA Limitd (FA31mm and FA77mm), the VL58mm is extra-ordinary (IMHO) in poor lighting conditions: e.g., fog, smoke, smog. In these difficult conditions, I prefer the VL58mm, over the FA31mm, for example. Lastly the VL58mm is superior in very-very-low light when the AF does not lock. The MF ring of the VL58mm is very smooth and easy to use, superior to the MF ring of the FA Limiteds.
Compared to the FA50mm f1.7, the VL58mm outperforms so much the FA50mm f1.7 that I do not use the FA50mmm anymore. Compared to the A50mm f1.2, I prefer by far the colours and sharpness of the VL58mm f1.4. I use the A50mm f1.2 (over the VL58mm f1.4) in rare situations when I need to shoot fast (at high shutter speed) in low light.
I shared a subjective limited comparison between the FA31mm and VL58mm during a trip on the Great Barrier Reef:
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/209251-fa31...mm-f1-4-a.html
The VL58mm f1.4 was my 2nd lens purchase, after the DA18-250mm. The VL58mm is my most used prime lens and I regard its purchase as my best investment. As you can see, I am biased for the VL58mm and I strongly encourage you to get one.
I hope that the comment will help.