I think I found the first crack in Apple’s hardware compatibly with FCP7.
We came across a very odd issue where newly created prefs would Edit to Tape as expected but after a custom window layout was loaded it would insist on rendering the entire timeline before outputting. Turns out having the Video Scopes open when attempting an output was the cause. Close the scopes and the rendering requirement vanishes.
Maybe this is due to the dual graphics cards? Are there more issues lurking?
I always changed the screen layout to standard without scopes to do a playout to tape in the old days. I don’t think you have discovered anything new b ut changing OS and hardware will bring out small differences like that.
But as has been said, it is sheer luck that FCP7 works on Mavericks or Yosemite. There is no attempt at Apple to maintain compatibility with FCP7 so OS updates will break it terminally one day or render it unusably unstable.