So this ticket is basically asking the VirtualBox maintainers to take a reliably working part of the product and to port it to a different compiler, retest everything, fix any bugs caused by differences between compilers and update the build process as well. So as an end user of VirtualBox, I would either notice no difference at all (BIOS isn't a performance bottleneck really), or will get a regression due to bugs.
With all due respect to the free software community, I would say switching the compiler used in a stable product without an apparent improvement noticeable by the users would not serve their best interests. It is also a bit strange to see it not in a form of "hey, we really care about free software, so we ported this part to use a free compiler, here's the performance comparison, here are the unit tests, here are the build instructions", but rather in asking someone else to spend considerable effort on implementing this.