i noticed in my car that the right side of the gearbox has a different synchronising mechanism than the left. each car would be different but they all have this force differential to work out.

is S3 going to model some things like that? (maybe it's not a realistic race transmission, and racing gearboxes are just free of manufacturing defects / ridiculously expensive.)

I have no idea what you mean. With a constant-mesh gearbox, isn't the moment of inertia of the internals the same regardless of what gear you're in?

I belive what he mean is thing that when you shift to low gear at high speed with pushed clutch real car usually produce some kind of awkard noise or you are unable to shift at all. Because somekind of "flywheel" (not real one) need to speedup a lot, which requires a lot of energy and a lot of force on gearbox. LFS doesnt simulate that thing, but it isnt really noticeable in normal driving conditions, but it simulates clutch weight which is more important at normal driving.

there's a different mechanism or flywheel for synchronising 1st and 2nd than for 3rd to 5th?

if i tried to shift my toyota matrix at lightning speed, surely the synchronising mechanism is too cheap to have the gear speeds matched up in time and the shift would fail, even if the clutching had been properly done.

of course i say this will all the confidence of zero knowledge of how actual manual transmissions work i just felt my transmission do something funny when going down into 2 one day... kind of like how sometimes putting the stick into R can cause a clunk even while the clutch is down, if some bearings are sticky or something.