But I know from the past that the GNU GCC compiler is not the world's most efficient. Right?

If you have used older versions of GCC you may well have formed that opinion. GCC has always been good for development, but older versions where not so efficient for runtime use. That changed a lot with the release of version, 3.0. Version 3.2.3 (the current basis for mspgcc) produces code that is very competitive in both size and speed. The mspgcc uses the facilities optimisation provided by GCC well, and produces very competitive code.