HC-05 is build around a programmable Bluetooth transceiver, the achievable USER rate depends on hardware capabilities, crystal frequency and firmware. From the HC-05 user perspective It's not possible to increase the rate above the value implemented in the firmware. Serial Bluetooth adapters with higher supported rate are available on the market.
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In additon, recent HC-05 modules are supporting 921.6 and even 1382.4 kBaud.
Additionally:
* payload of UART is 80% (10 bits per byte, no protocol)
* But but Bluetooth includes additional data for protocol. So payload may be less
I'd say, there's no guarantee for a specific continuous data rate on a radio channel. You need to implement hard- or software handshake to prevent data loss. I can tell that 921 kB is typically working.