1- OWC makes solid products.
2- Most all optical drive burners out today are pretty much the same.
3- Toast doesn't go in between FCPX and a Blu-Ray burner. FCPX will see any Blu-Ray burner any other apps sees. That is OS X, it sees the burner, and makes it available to all apps. Toast simply offers an alternative (a slightly more solid one) to burning a Blu-Ray movie DVD, and offers more menu choices and options. But if the operating system sees the burner, all apps written to interface with Blu-Ray drives will see it.
You do NOT want to burn optical media too fast. That can cause bad burns and player incompatibility issues. Even back in the NTSC-SD DVD days, I'd rarely burn anything faster than 1x or 2x, just to be safe. Faster burns introduce more errors. Open a bottle of wine, relax, it's Blu Ray, which is never fast anyway.
I know the box says it is a 14x burner, but I'd never even burn a music CD at that speed. Optical media burners shouldn't go that fast, totally ridiculous with the danger of bad burns at those much higher speeds. IMHO.