>If you can find out if Metal will be supported on earlier versions of OS X, that could make it a lot more interesting for me to look into.

According to Apple OS X adoption rate is very high, 55% of all mac are running the latest OS X. I thing you shouldn’t worry about that, after all upgrading OS X does cost $0.

Also Rav Dhiraj from the GPU software division states at 8'40"
Metal is supported by all Macs introduced since 2012. (Nvidia, AMD and Intel)
https://developer.apple.com/videos/wwdc/2015/?id=603


>Well, I hope you're aware that the Foundry and Adobe have considerably more developers working for them than I do. That makes it feasible for them to work on Metal as a side engine instead of their only supported one. Please keep in mind that I'm only one single person doing all the development work for MoI and that makes it difficult for me to do exploratory side work, I need to focus my efforts on the main line work that will be used by the most people.

Michael I am well aware of that and thats one of the reasons of me proposing Metal, (once the initial transition and teething pains are past) will actually be easier to maintain, with less time worrying about various driver and hardware compatibility issues.