I also have N5 and demo of N6 and can do my mastering in it.
Only thing I really like in wl7 is the Smart bypass function, which shows the difference between original and processed master. Which I think is a must have.
In N5 or N6 you have to make a separate channel and match loudness manually, which takes time and is not as accurate.
If someone develops a vst plugin for such purposes, which can be used in Nuendo then I would do it there and use WL only for DDP creation.
Regards
Denis
www.mastering.lt