Im up to 30K miles now and have seen a massive improvement in MPG and also performance in the last 10-15K - although i have a very heavy rightfoot and enjoy the car so much. I usually get about 370miles a tank
and usually fill up at Tesco
Ive been using BP a lot more recently and break 400 everytime?!, but back to 370 with Tesco same driving style. It doesnt make a diff at BP whether normal or premium, but do notice a improvement in performance from an empty tank filled with premium, without a doubt.