This is my anecdotal evidence... YMMV

On Windows I found that the MSVC compiler from the Windows SDK was slower than the MingW GCC in the Qt SDK building the same source (using the QMake default options). The MSVC compiler always seemed to produce smaller executables (presumably because it assumes the presence of more on the machine).

For my desktop application runtime performance differences were not noticeable. Unless you are doing some constant and mathematically intense calculation I doubt that the performance differences in generated code are likely to be significant.