I’m Jerome, CTO at CME.
Indeed the buffers for both RX and TX are 256 bytes.
This cannot be increased too much due to in-IC limited memory.

Your suggestion to forward incoming SYSEX directly without buffering is not possible, as receiving and forwarding are done in separated threads. Incoming data require buffering.

In order to understand, in which direction are you receiving these long SYSEXs ?

USB Rx to BLE Tx
BLE Rx to USB Tx

Please let me know so I can try to see if buffers could be increased at least in one direction.