How does -Ofast increase size compared to -O3?
I had in mind -Ofast or -O3 versus -Os (which is used by default when building ReactOS). Now I realise you probably meant -- consider replacing -O3 by -Ofast...
BTW, will it be possible to compile kernel-mode code and user-mode code with different sets of options? Somebody might want to build user-mode stuff with more optimization then it's allowed for the kernel.