It would be nice to see a furthur discount for say 20+ and then 30+ and 40+ and so on. Also a little discount for renewals wont hurt for say those who hold 50+ and you could make the renewal fee just $20 for them or even lesser.
I believe this would result in increased sales and most people who tend to distribute their new launches between 3-4 different softwares available will have no choice but to stick to Xenforo for every new forum that they launch once they hit the threshold that provides them a decent discount on purchasing the liscence and also renewals.
To avoid abuse you could restrict the licenses to be used only for domains that are registered in the same company name or individuals name. You can also restrict transfer of highly discounted licenses or charge the difference for transfering equal to the discount that was provided at the time of purchase. I am sure you can work out a set up to make sure no one is able to abuse the discounts they get.
If this is already in the pipeline then I would highly appreciate a quick implimentation.