I use my car frequently on track days, with engine oil typically reaching 110-120 degc. But what happens to the gearbox oil? It is changed according to the Subaru service intervals, but lately I've noticed that the gear lever feels harsh and needs more force to operate. Any opinions?

I would suppose that a track-day could well elevate your gearbox oil's temperature outside of it's normal working range, so it might have degraded in some way as you say.

However, I'm not an expert so this is just an opinion. Won't do any harm to change the oil and see if it helps - a lot cheaper than having the gearbox off!

I recently had the old MY99/00 5th gear problem sorted and while they were doing it I had the oil swapped for Castrol Syntrax. Made a big difference - I can actually get into reverse on the first try now!

Is it the gearlever linkage or the syncro rings have polished?
Have seen a few boxes where the syncro rings have polished up with quick gear changes, thus not allowing them to slow down quickly enough to permit the gearchange.
Without a strip down and checking, no real way of telling. You may get a slight change in box oil colour, but it needs some serious thrashing to do such.

Rectification for polished syncros is simply to remove and abrade driven surfaces with wet n dry paper, similar to "lapping in valves", creating the same finished effect.