Though surely, the TX opto should be a TCLT1009 (CTR of TCLT1009 = 300%) and not a TCLT1000 (CTR of TCLT1000 = 50%)?
Also, surely the RX opto should be a TCLT1000, which it indeed is.
The reason that the RX opto should be a TCLT1000 is because it has a lower capacitance seen at its emitter pin which means the rise time of the emitter can be quicker. Do you agree?