Does one need more than 5 minutes continuous, would 30 minutes at 90% suffice?
I've spent over an hour in the hover at max continuous, so no, it isn't enough.
The 20 minute limitation clearly is battery life on the Extra which is why I asked about a generator as you suggest for ETR - if the answer to that is cooling then you would have to keep 'resting' your ETR to stop it overheating.