I've had a few Big Macs lately, and from my perspective the meat is perhaps the least consequential part of that sandwich, both flavor and texture-wise. At one point I asked for 4x extra cheese (gross, I know) and after the first few bites, that too seemed to be pushed aside in the palate by the sauce. I can almost guarantee you I would not notice a difference if they swapped the beef for beyond meat.
In my example, it was about 1/2 or 1/3 lb worth of meat, with minimal fixings, so at least some places are definitely using it in a way where the flavor/texture of the 'meat' matters.
