I had quite a large number of people reply privately to me on this (see below), but a singular lack of vendors (just one) who had anything to say on the subject.
So, before I conclude that there is only one vendor in the world that supports L2TPv3 at anything close to gigabit speeds, I thought I'd ask again, more explicitly:
If there are any vendors interested in selling L2TPv3 boxes, and they'd like the list of potential customers who have sent me mail about this to find out about them, then they should feel free to send me a sentence or two about what their boxes can do so I can pass the information on.
Suggestions and recommendations from operators would be also wildly good to hear.
Thanks!