I use a B&H 10x ND Filter on the 17-40, there is definitely heavy vignetting @ the 17MM end but it's easily correctable in Lightroom - although you'll have to go a step further than just using the built in profile corrections for the 17-40mm, I had to go into Manual Vignetting on top of that and adjust to +40. Here are a few samples, first one is raw straight out of camera, second is processed and vignetting taken out:
I bought the Hoya Neutral Density x400 HMC (9-stop) for my 17-40L in the native 77mm size, and I don't see much if any extra vignetting at 17mm on a FF body (6D). I just applied the standard Lightroom lens profile and the corners looked fine at f10-f16. Vignetting may be aggravated at larger apertures, but I only use this "black glass" filter for deep landscapes anyway, so it's irrelevant for my use.
The aluminum Hoya mount does seem ever-so-slightly narrower than the typical B+W brass ring, so that may be the difference (but I haven't measured it). I don't have any experience with the Heliopan filters mentioned.
Definitely recommend trying this sort of filter. Really opens up interesting photographic opportunities. Here's a recent example:
Canon 6D, 17-40L at 17mm, ISO100, f16, 30sec: