On the FT-450D:
CAT RTS: Disable
CAT TOT: 100
CATRATE: 4800
In Omnirig:
Rig type: FT-450Dmod
Baud rate: 4800
Data bits: 8
Parity: None
Stop bits: 2
RTS: Handshake
DTR: Low
Poll int., ms: 100
Timeout, ms: 100
Not perfect, occasional lag / dropout, but pretty decent.
Omnirig is version 1.16, serial port adapter is VID 10C4, PID EA60 and reports as Silicon Labs CP210x USB to UART Bridge.
Driver provider: Silicon Laboratories Inc.
Driver date: 19/09/2016
Driver version: 6.7.4.261
FT-450D firmware version 0244 (hold down Mode while powering on)