I recently made this expensive mistake. The wide angle adapter doesn't fit properly over the hood bayonet on my 24-105L, and whilst the less dramatic 0.9 ND and ND grads seem OK, the 2.1 has a very difficult cast, made much much worse if I stack the two to make an ND 3.0 i.e. 10-stops. I can't correct these casts using custom white balance, and having spent quite a while playing in lightroom last night with them, the closest I get is still quite some way from an unfiltered sample I took.
Oh, and I find even the HiTech wide angle holder and adapter slightly vignet at 24mm on full frame. This may differ on a 17-40 for instance, but I doubt you'd get the full 17mm - something to bear in mind as you're 16mm equivalent on the 10-22mm
Looks like I'll be selling the lot (not cheap, cost £200), and shelling out for Lee after all. If you can afford Lee, go for it first time...