if you can afford the extra £4 - 5 a tank do it, if not your not missing out on stellar gains, i use only v power and its a bit smoother over normal shell unleaded, if you drive like a granny then normal unleaded seemed to me to give better mpg in my 2.5i - mid thirties, but when you put your foot down every now and then it drops to 27 mpg. on v power i can hammer it like a mad man and still get 27 mpg, or do a mixture of normal driving and thrashing it when the opportunity arises and still get 30 mpg.
all depends on how you drive, or if you feel the extra cash per tank is worth it.