Anyone know if it's possible for any of the emulations to have a 'true' monophonic mode? I.e you play one note and then play another and then take off the second note and the synth returns to the first note, so you can therefore do really fast thrills?
No, at the moment this is not possible, when you press a second key for a mono synth then the existing voice will be allocated a new midi note and no history is kept. It is on the roadmap as a rework such that any synth that starts with a single voice does have this functionality, I am still considering what do to with releasing notes, whether to give another key_on event for historic notes - I don't think this is how they worked however some people have argued that it was.
If you want to move this forward faster then you might want to open a feature request.
Actually open as Bug ID 2120507 against monophonic note assignments being damaged.