@AdderD
If it affects primary the DUE, I think the DUE section would be better. Still for the UNO's under us this is interesting too 
@Nick
Never reviewed the details of the wire/twi code but if I see this I really think a rewrite could save a byte or two
1st thought is why not reuse buffers on the 2 levels?
2nd using malloc could still be proposed (as long as we not free the buffer, or is free() fixed?)
3rd do we really need a separate RX and TX buffer?
food for thought ...