Re: ND filtering advise needed...
 

Gabe,
No problems with your approach. Yes I do use a matte-box (plus rails etc) most of the time, in which case using 4x4 or 4x5.6 filters is a natural. My point is most pertinent to people who don't want to load up with a matte-box and prefer threaded circular filters. That is, really you won't need to be swapping NDs.

Re: ND filtering advise needed...
 

If you've already got the filters, you could use a clip on matte box. They fit onto 80mm cine lenses, but you can fit adapter ring to your lenses, also without the 80 mm adapter ring the newer 95mm dia cine lenses.

ARRI Group: Lightweight Matte Box LMB-3

You can get them for larger filter sizes.

Re: ND filtering advise needed...
 

I have seen some good comments about the Genus variable ND filters, but they don't have a thread on the front of the filter. However I noticed on the Genus website that there is a neat little short handle that is screwed into the rotating filter that I consider a major advantage. I have done a lot of browsing on Genus variable ND filters, like B&H, EBAY, etc,and no one's photos of the filters I have seen show that handle, which is unfortunate.

Genus ND Fader Filter - Filters - Genus Video and DSLR products

I have tried attaching handles, gears, etc of my own and found that the clamping action of these attachments can cause the rotating filter to bind, so I haven't had complete success.

Re: ND filtering advise needed...
 

The Genus rotating ND filter pair has a removable short handle which ships with the filter. I have the 67mm version for the kit lens (that lens in so-so) but the Genus does not seem to hold a lens cap - it needs a reduction ring perhaps to hold a cap on! Almost a good product...it does its job well otherwise, allowing the option of using the desired iris setting when shooting in bright sun.

Re: ND filtering advise needed...
 

I have a Genus and it works well until you get to almost completely dark, then it makes a soft pattern, not a uniform light change, and becomes unusable. And the little screw in handle lasted one easy shoot. Other than that it's fine.

Re: ND filtering advise needed...
 

Also, while solving an obvious problem common to polarizers as well, I would think the handle also interferes with using the lens' shade or a matte box. YMMV