Currently, for an Instrument Track which has the Instrument Output set to an external instrument (HW MIDI synth), the Volume and Pan faders do nothing.
If an external instrument is specified as the Instrument Output, these faders could optionally send MIDI CC 07 (Volume) and CC 0A (Pan), to the MIDI channel and port that is specified in the Instrument Output of the Instrument Track. This would allow the faders to control the Volume/Pan of the external synth.