I think that there are to dimmensions to consider here: first of all, the Viewing Panel at B&O concluded that you need to cover the field of vision with the picture to make the 3D experience, well, a positive one. And they landed on 85 inches for a living room. That is a huge screen no matter how you look at it The other thing is the simple fact that all high end panels being made today are 3D.
So the BV 7-series is too small for 3D and LCD is not the best panel type for 3D (plasma is better due to the different technology used) - but unless you have the cash and the space for a 4-85 (or the 103) you should hook up a 3D projector to your BV7 3D TV to fully enjoy the 3D experience. The panel on the 7-55 (mk2 vs mk1) is way better for 2D content, but I have not seen a 7-40 3D at this point in time so I do not know if this is the case here. But if it is as good as the panel in the 7-55 mk2 then my recommendation would be to go for it (coming from someone who is replacing their 7-55 mk1 with the mk2 now )
I do belive that more 7-series owners should investigate the installation of a motorised tab tension projector screen and a decent projector. It is so easy to hook and control with a BV7, and so elegant.
Roger