Feel free to start creating patches so we can get an idea of what the changes would look like. Hopefully it's not that dramatic.

Be very careful making performance claims without benchmarks to back it up, and ideally against multiple sets of hardware (MSVC is designed and tested to perform well across a range of processors, often by engineers who work for the manufacturer - intuition would suggest that an open source compiler is probably not 30% better all the time). Don't focus on the number right now, but do try to collect the justification before you expect or encourage others to do the work.

Since the ABI is compatible, there should be no problem enabling extensions to be built using this compiler (assuming someone is willing to become a distutils maintainer, as there are currently none). You don't need to ask here to create a third-party library that enables this.

I haven't heard any complaints about access to the compilers being an issue recently, so the only reasons to switch the interpreter itself would be source compatibility (essentially, the clang clib is better than our custom Win32 code) or performance. But we need a positive reason to switch support, not just the ability.