I had my G5 up to 80mph, briefly, and it had a bit more available. It felt a bit unstable. It's happy to do 70 - 75mph for a short run up the dual carriageway, but I wouldn't want to do it continuously for a long run. It'll do 60mph all day without complaint, especially since changing the exhaust, which increased the sweet spot by a few mph.
I have never got less than 82 mpg - UK gallon. Best was 87 mpg.