This is what I am planning for my own live-fire E-11... keeping with the original t-track and attachment method. Unless you are mag dumping a full sterling magazine, I don't foresee enough barrel heat transferring to the shroud to melt the plastic. You could rapid-fire your 3-rounder and probably still be fine.T-Track:
While we all strive for accuracy, there have to be made compromises when trying to keep a prop functional...some more/ some less obvious with regards to the finale appearance. Plastic t-track is relatively inexpensive, so I´d simply get two sets of Roy´s "economy" track and get to work...you can always swap to a metal solution, if the plastic track fails.
Without having fired a Sterling myself, I don´t know anything about the physical stress, that the vented barrel jacket is exposed to (heat/ pressure etc.), but I can think of a way to have either the plastic and / or metal track secured to the receiver...will PM you a scetch of my idea.
I would never consider modifying an authentic Bergmann. However, as Odiwan72 said, it would be nice to display the original inspiration next to a replica blaster.