The PathScale compilers (now owned by SiCortex)
aren't explicitly for Win32 programming, but they do
provide a high performance compiler suite for x64-based Linux systems.
They claim to be the "highest-performance 64-bit C, C++ and Fortran compilers for Linux-based environments,"
and their SPEC scores are very good.
Unfortunately, how to purchase of the Pathscale suite isn't obvious,
since SiCortex is a hardware vendor. They say they ship the PathScale
suite with their hardware, but don't seem to have a link for buying
|PathScale||last update 7-26-2009||