i remember 5th gear doin a review on petrol between normal petrol V-power and bp ultimate 
it was over 3 cars, 1.2 clio, mk5 golf gti and a subaru impreza
none gave any performance difference for the 1.2 clio 
about 5/6bhp for v-power and 3/4bhp for bp ultimate for the golf gti 
and about 10/15 bhp for the v-power, i cant properly remember but it was the higher the performacnce of the car the better the gains from the pertol 
v-power beat BP on every occasion