I don't have a variable but I've a buddy who uses a Singh-Ray and he loves it and he's never mentioned any distortion issues. You'll certainly will pick up some vignetting but that's fairly easy to correct. I have a couple of standard B+W's ND's and truth be told, I thought I'd use mine more than I do and now with the D800 and it's 3 levels below ISO-100, I don't know how much I'll be using my B+W's.
Are you sure you need a variable vs. standard ND filters? They're not cheap, at least not the good ones, that is.