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Basically only Gentoo/Arch users can who compile all the day can use different compilers/settings to improve speed. But they will never gain the time back they used to compile the sources on their own system(s). If you use a more generic distro then all packages have to be compiled to work on all supported systems. I don't think a 5%-10% gain is worth to create a specific binary, that's only important when the base speed is low which is very unlikely if you own a new system. A completely different thing is when you have to your own code and you want to run it as fast as possible - but then you have to do you own tests as no compiler comparsion will be accurate for custom code. I compile xbmc from sources usually but the reason is not the speed but that this app gets so many updates in a short time that using binaries from a release feels already outdated when it is tagged
-march implies -mtune. With march set to your cpu, it's pointless to add mtune.
but I read:
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. While picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any code that cannot run on the default machine type unless you use a -march=cpu-type option. For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines.
The choices for cpu-type are the same as for -march.
OK, I guess I got it backwards. march does more than mtune.