not a graphics programmer but here goes:
- opengl on tf2 works by a layer which translates directx calls to opengl (something like this, though that seems to be for dota)
- if you want to run it on metal you'll need to translate from directx or opengl to metal. opengl to metal wrappers seem to exist because apple deprecated opengl on iOS. MAYBE its possible to use these on mac. writing your own without help will take years.
- iirc a lot of the performance improvements in metal come from the fact it's a lower level API (similar concept to vulkan) and so provides more granularity to rendering engines. dont expect to just glue it to tf2 and expect performance improvements as the game will still try to use a directx api
- if it's like windows, the game is largely CPU or memory-speed limited. i wouldn't expect GPU speedups to help you much