Hello peterjcb,
ND filters are handy for a number of things, 2nd only to CPL's for digital use. Pretty much every other filter 'effect' can be duplicated with PP.
They screw onto the lens front, with whatever diameter thread that lens uses. I looked up the DA 15Ltd, it's 49mm. FA 31Ltd is 58mm (filter diameter size), so they're widely different. Some people would buy the larger size and use step-down rings (reducers) to accommodate the smaller sizes. Others have filters for every different size they need. Cost is of course a factor, but so is remembering to bring' em all and space in the camera bag.
I see you have other DA's and FA's. It might be easier to find a couple of similar-sized (filter size) lenses like 49 or 52, two common Pentax sizes. Then you could easily use the 52's with one step-down.
There's also a problem with wide-angle lenses and filters, called vignetting. That's when the filter 'cuts off' the outside of the image, darkening it.
Either way, get the best you can afford. Mounting a $10.00 knock-off onto a thousand-dollar lens seems pretty dumb to me, but opinions vary. Anything you put in front of the lens will degrade the image quality somewhat; Good CPL's more than make up for any tiny I.Q. loss with the enhanced saturation, increased contrast and glare reduction. A lens hood offers many of these qualities also. A hood and good CPL together? Real photography, not snaps.
Enough lecturing. I don't know of a ND test, but here's a link to 'LensTips' a lens-review site. This is a summary of a CPL test. They also have (under 'Articles") a similar test of UV filters. The same names appear at the top of both, so it's safe to say they probably make high-quality ND's.
Polarizing filters test - Introduction - Lenstip.com
B + W
Marumi (Marumi also makes Fujiyama, another high-placer in CPL's)
Hoya Pro
Slightly farther down you'll find Kenko, Sigma and a few others.
Last, for ND's, you're losing light so the shutter speeds will be slow. Tripod, cable or electronic release (or, 2-second timer).
Good Luck!
Ron