Agree - I get about 30 mpg with my V6 190, if I'm careful, on petrol and about 22 mpg on gas, which equates to around 42mpg, so I would suspect 50 mpg from a V8 coupled with an 'enthusiastic' driver would be lucky to achieve 30 overall?
I'd be interested to know what other 260 LPG users have found so I can start saving up for when I find my Unicorn!