If you can't get half-duplex running with RS-485 you won't get it running with the buffers, the software will be exactly the same.
Can you post your code? My guess is that are not waiting for the bytes to go before to turn the line around.
With the new interrupt driven Serial (and lack of buffer management methods) that will be difficult.
I feel that this will cause problems with many programs, the new buffered output should have been an option I think, extra parameter, ability to 0 the buffer size or whatever.
Rob