Not in the case of big software projects (and your C compiler is undoubtedly a big software project). Its taken about 10 years and millions of man hours to get

Not in the case of big software projects (and your C compiler is undoubtedly a big software project). Its taken about 10 years and millions of man hours to get clang/llvm on a performance parity with gcc. Any new C compiler has to either just be a C -> optimised C compiler (in which case why not just write an LLVM pass) or duplicate all of the optimisations which exist in LLVM/gcc.