Yes, that is correct. All mesh based products have large amounts of over head. Part of which is due to the way routs are discovered and part is due to the amount of hops needed to get the data sent.
If I recall, the serial buffers can only support up to 1 full RF packet (256 bytes) per buffer. So 256 on Tx and 256 on Rx. Just note that there is no handshaking that keeps you from buffer over runs between the two modules. That is, even if you are using hardware flow control, that only keeps the data from flowing out the UART of that specific module. It does not keep a transmitting radio from sending any data on its UART over the RF port.