Let me get this straight. You have access to a rental house full of lenses, yet you're complaining that "in the real world" you could only compare the 17-40 to a limited few?
So again, as someone who is wanting to buy something now...your review didn't help me. I'm sorry if that flies in the face of the Justin lovers (your last name isn't bieber is it?). I am truly undecided, but have pretty much ruled out the 17-40L. But then, I have specific aspects I want for the lens, and it need not even be a zoom (although obviously zooms are incredibly convenient, and my usage will not be "single purpose").
It's "Van Leeuwen." Though someone once tweeted me a message intended for him. We're both Canadian but I'm better looking.
The Nikon 16-35 f/4 VR is a remarkable multi-purpose wide-angle zoom, sharp and able to handle the resolution of the D800, so a 6D is no contest. Don't know how you feel about adapters, I'm afraid to say I'm not crazy about them unless going for something truly unique (like the Nikon 14-24 f/2.
. I think Canon can do better than the current 16-35 too. Their recent upgrade of the 24-70 f/2.8 proves that they're capable of some zoom awesome.
I've yet to spend enough time with the 17mm tilt shift on a full frame body, I've used it on a 7D (kind of a waste, but it's what I had at the time) and my old 1DMKII (RIP). 14mm f/2.8 is a really cool lens, depends on what you need, but if you love wide then that's a choice piece.
I have zero experience with Rokinon, their 16mm f/2.0 sounds neat. Maybe I should contact them and see if they're interested in having us review them here? Would that interest you (or others)?
Been working on getting Sigma stuff, nobody rents them here because you can't get the lenses serviced in a decent amount of time. Past QC issues made that a non-viable business choice, thus the first party offerings, and specifically L series glass. Though I have been borrowing and buying other EF lenses to help round things out.
While we're considering all options: what about micro 4/3rd cameras? An entire line of fast glass across the focal spectrum. I'm wholly unfamiliar with it, though, since I can barely afford one lens system. Don't get me started on medium and large format. My interest there is mostly in portraits.
I'm missing others too, I'm sure, there's just so much out there.