What the heck is the "BD-25 AV asynch bug"?

Never heard of it. All my BD rips, whether MKV or ISO, have played great on Q5.
Re: RPi2, it can't bitstream, but can decode and pass HD audio losslessly to your AVR. So, Pi2 is doing the decoding vs. your AVR. This is 90% just as good. Only aspects you're losing are some dialnorm metadata, and mainly no ATMOS and no DTS:X.