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?