I somehow managed to compile both mess and mame with march=core2 on OS X, with Xcode 5, no problems at all.
But you're right RB, an unoptimized build is slightly faster.
eta: No, it doesn't crash when entering the castle, but it crashed when I entered a room inside. Also had a crash with an unemulated opcode message when I jumped into the water. And again when I left the intro running while setting up the controls.
