When I worked in the chemistry department of my university, we did some compiler benchmarks on Pentium 4's, AMD Athlons and even some Alphas (gcc vs. compaq compiler in this case). We did this at a time where icc was being touted as best than sliced bread, but found out that while it mostly outperformed gcc on the Pentiums, it did so by a small margin. On the Athlons gcc was faster across the board. On the Alphas, gcc was also better (except on fortran code, where compaq compiler was way better).
And this was gcc 3.x. Since then gcc has evolved quite a bit, and I wouln't be surprised to see it take the performance crown everywhere if I repeated these benchmarks today.
To be fair tho', $vendor can concentrate 100% on $processor, GCC at least tries to be cross-platform.