There's a request for deletion of this package, I don't know where to respond to it. It's been argued that -O3 and local tuning should be part of the local makepkg.conf.
The package description fails indeed to point out all optimizations, but even if it was only those optimizations, this is simply not how the original gcc's build works. Some makepkg.conf flags are suppressed and gcc has specific mechanics that simply adding those options to the makepkg.conf won't build an optimized gcc the way you expect.