If you use a prefix then it might be suitable to set up a neighbor zone to the MCU and match that
with the same priority then the local zone with the prefix registration and strip away the 80 to match
it on the mcu.
But it all depends on how your all over setup looks and might even differ what services you support
(tms booking, pre defined meeting rooms, multiway, ... if presence is neded, and so on, ...)
If you use the VCS-C you have 100 traversal call licenses anyhow. The binding of the media might be more critical.
What is the reason that you need to have sip as the default call protocol? maybe using h323 directly fixes your issue as well :-)