Former R27 & Mk1 V6 owner
As for fuel, it is pretty bad, 150 miles from 23.5 litres (5.22 gallons) is around 28mpg! Fairly short journeys through the week, then at the weekend it gets a few decent long drives to get it up to temperature and then gets a bit of a thrashing. Not sure why it is so bad, just a revvy little engine.
There is a decent amount of torque under 3krpm, but it sounds nicer at 6.5krpm!
TBH that's not that bad - it's not a tiny engine at the end of the day, it's a 16v and IME they're always thirstier when thrashed. Also like you say, it never really gets warmed up during the week - and engine economy is a lot worse when they're cold. So if that's coupled with some town driving and been turned on and off a few times a day, it will be poor. Then you say yourself, you like to give it some at the weekends when you're driving for fun. When I had my 206 at uni, it used to do about the same, cos it was just loads of short trips and town work, then the odd blast. Tank seemed to just disappear heh!
Using a 172 in the same way, I wouldn't be surprised if your saw mpg a bit worse - as the 172 is a heavier car, with a bigger engine, but more importantly than the engine size, is that it makes more bhp/litre and more power means less mpg heh!
£500 for the insurance as well is a bit harsh! It only cost me about £150 more to insure the V6 (group 20R plus high risk) than the 197 (group 15)!