Well, here's my use for the thing - I shoot color neg in it, and for the most part I scan the negs. Therefore, I want to take the Detroit option - there's no subsitute for (square) inches. Also, I've seen excellent condition 250s going for $125-200. Not much more than a 645 back for the RB. To me, a lens is a better solution than a back plus a finder mask and/or a teleconverter, because it's only one thing to worry about changing on the camera, as opposed to two or even three things. And it's much easier to set down a teleconverter or a finder mask and forget where you put it than a big honkin' lens.
As to tripods to handle whatever lens, you're right, I've got that covered. If the Induro wouldn't handle it (which it will), then the big Gitzo 1420 series aluminum legset will. I could probably successfully defend myself against a grizzly with the Gitzo. And yes, small apertures don't concern me in the least, although for someone who shoots mostly large format, I tend to work in the wider open end of my aperture range than many. I'm not an f64 junkie. But yes, f9 is nothing to be afraid of. I've got several lenses that have that as a MAXIMUM, even without teleconverters and the like.