That helps alot! I think ill go up to 20s for sure but defs not dropping the ride lower, we have 1 tuner (horsepower factory) who advertise the tune but I spoke with em and they use a mix of spark plugs, air intake, catback and unichip(I think) to gain you about 150 hp for close to $5000 which is outrages.I've gone up to 20s on mine, and I'm in Perth. I daily drive mine all around with no issues. If you're worried about potholes you could stick to 19s and just go wider but I think at most just don't splurge on super expensive wheels for your own piece of mind.
I dropped mine on H&R springs, I would suggest you *don't* do that if you've got lots of uneven terrain. I can scrape the odd Macca's carpark if not careful so potholes would be tough.
I'm on the stock bi-modal too, completely agree however I don't have an answer for you.
If you want some more speed the most reliable seems to be the JB4 piggyback ECU. If you'd like a tune and want to stick local you want Unichip. They're actually in Victoria so you might have some luck.
Hope that helps mate.