I probably cant help with the specific gorge, but like all things it's pretty subjective depending on a whole heap of variables.
My experience with wetties comes from a range of different uses including surfing, waterskiing, packrafting and canyoning over 30 years or so. All of them across a range of temperatures from warm to very cold water in winter. Waterskiing mid winter on the Hawkesbury at 6am has probably produced some of the best hypothermia I've experienced due to windchill but not really relevant to the topic. Dry suits for the win these days.
Surfing and canyoning in old style wetsuits 25 years ago was definitely a different experience to what it can be today. The technology in new surfing wetties today is a whole different ball game. The fatigue that came with the old style wetsuits along with the fact that they weren't all that warm had me searching for better wetsuits all the time and I'm due for another new one soon. Canyoning in my opinion needs a good stretchy surf wetsuit due to walking and scrambling all day. I have a good 4mm for prolonged cold water use but it is so tiring that a good 2/3mm wetsuit is wins every time.
I wore a 2/3mm for 6 days straight on a packraft trip but had to pull the top down and tie the arms around my waist because it was too hot. Farmer Johns would be good not as easy to find these days. Canyon water has it's own special kind of cold but plenty of people I know still travel fast and light in shorts and t shirt through very cold conditions, but they are experienced and are fairly hardy souls.
In terms of the ultralight question and how you would keep yourself warm...take the extra set of clothes. If you have been as cold as you say on previous trips then what's the point of doing it to satisfy your ultralight persona. Sleeping in a wetsuit is a gamble based entirely on the overnight temp. One set of clothes can be thermals and lightweight kit that really wont take up much space in the pack. Being hungry wont help your body temp either. If you cant carry a few hundred extra grams, something is wrong. You are there to enjoy the surrounds and should be looking outward, not shivering or wondering if you should have taken more kit etc.
One thing I do like about wetsuits is the extra padding when scrambling or being battered on a river somewhere.
I'm keen to know how it went anyway and what type of conditions you encountered so keep us posted and have fun.
