They're all usable wide open and yes, some are better than others. I have to wonder though, Why, with a camera like the K5, you believe the DA40 is not Fast enough? If you just want a new toy, I can appreciate and understand that. Be aware that the 50 f1.7 options are out of production so your only choice there is Used. To get one at a reasonable price, you may have to hunt around a little bit. I'll guarantee (almost) that the people who bought Berts lenses, won't be reselling them for $200, IF they resell them.
All of these faster than f2.8 lenses are likely to have a few issues on the more modern cameras because the cameras just aren't set up for them. The typical digital Pentax lens tops out at f2.8 (or f2.4 in the case of the DA35). The exception to that in the Pentax lineup is the DA*55.
The FA50 f1.4 is a nice lens. I can't take away from the optical quality but I've never cared for the Build of the FA primes. They feel cheap to me and I hate the focus rings but that may just Be me.
The DA*55, I never had issues with the focus speed. I don't know what you are after in that respect but mine performs just fine. If I missed a shot, it is because I wasn't prepared at all for it. The lens completes your weather sealed camera, is quiet, has an AF/MF switch that I find quite handy. It Is bigger and heavier than the FA counterpart though.
If that range is what suits you, and the DA*55 is in your consideration, also consider the FA43mm f1.9 Limited. Or if you want to go a little longer, the FA77mm f1.8 Limited. Both can be had, used for DA*55 money.
I would consider though before spending a bunch of cash, answering the question I opened my response with.