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.
 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