Did you try the same setup (two bluetooth input devices plus bluetooth audio) on a regular Mac or, say, with the same device but using "proper" drivers under windows? It should not cause any trouble, but I once also hat similar problems using three bluetooth devices (one of them bluetooth speakers) on a regular iMac. Although it did not result in instant reboot, but the stuttering was there, too.
There are some posts that if you use the Mac Pro profile (which I assume you do if you have an Nvidia card) the performance suffers compared to using an iMac profile. This would explain your experience with the benchmarks.
I have no idea if FCP uses the GPU for rendering? So there might be a trade-off. Maybe you could investigate if it is possible to use the iMac profile together with the Nvidia card when you use the Nvidia web drivers or something like that.