v2: Keep all default queue limits.
I tried again with Trailmakers and P4G (which once required the size limit to be kept, but the game then changed their movie format and backend) and the unlimited buffering doesn't seem to make any difference anymore except using much more RAM.
That makes a lot of sense, thanks for testing.
This has been on my list of things to reëxamine for some time, actually. It was originally added because mfplat sources don't get starved, but I later found that multiqueue (and hence decodebin) has logic in place specifically to prevent this kind of starvation, so yeah, we really shouldn't have needed that.