bluetooth headphones and MIDI keyboard-how big latency?

Bluetooth keyboards can work fine, I know the new seaboard even manages it and that’s more data than regular midi.
I have no experience with them though, so check some reviews online for the specific unit you’re interested in.
As for the headphones, I would personally not risk it. I highly doubt they come with ASIO drivers, and they are simply not designed for very low latency so it would surprise me if they perform well.