Simply because I had the filters to hand from my days with "real" film, I did a couple of comparison tests between my *ist DS and my K-5, both unmodified, using the same lens/filter combination on each body. The K-5 was a little disappointing ... it simply doesn't respond with an IR filter fitted to the lens. even the live-view shows nothing, though it will work with a very dark red filter.
However
... the *ist DS is surprisingly capable, even with a "dark" IR filter it gives a good response. Of course, you can't see a thing through the viewfinder and there's no live-view ... so I use a clip-on multi-frame viewfinder from my old Russian rangefinder camera fitted in the hot-shoe.
Edit: Don't forget the viewfinder cover on the camera when using an external viewfinder ... it makes a big difference!
With appropriate choice of framing to compensate for the crop sensor we're in business
This set-up can also work well when using a pinhole !
There's parallax compensation to be considered for close-up work, of course, but with a 35mm lens on the *ist and a 50mm frame selected in the viewfinder, we're good to go for most scenery/landscape type work
There's a rather tired-looking *ist D in a second-hand shop in town that's a candidate for conversion, but it'd mean a further investment in CF cards, so I'm thinking about it.
Just my tuppence worth ... hope it helps.