That said, there's a finite amount of threads the compilers can use. It's 16, so an 8-core extreme edition i7-5960X (or the newer Broadwell-E 8-core CPU would do too) is the most extreme CPU you can use before you're starting to not use cores. You could use them for other things though.
I also remember someone wanting to build a GPU accelerated version of the compilers though which would speed up the compile process (especially VRAD) by a whackton, but that was years ago, and the project never even left the ground.