If you're thinking of buying it steer well clear cos it sounds like he drives it like he's stole it all the time - therefore it will have had a very hard life.
Mine was running around 300bhp and I didn't notice any change in my mpg. I averaged around the mid 20's but on a steady run could get it in to the 30's.
When mine was totally standard (apart from a cat back exhaust), I did a track day in it and averaged 9mpg. So his quote of 8mpg is realistic, but you would really and I mean really be pushing it hard to achieve that - and quite frankly if he pushes that hard on public roads he doesn't deserve a driving licence cos sooner or later he's going to kill someone! My 9mpg was achieved by keeping up with and staying ahead of some BMW M3's just to give you an idea of how hard I was pushing on a track!