read the lil sticker inside your petrol flap and u will be enlightened 
read the handbook and you will find that they can be run on 95 and lower, even 91 iirc. com-puh-ters make up for it. obviously running 98 will yield the best performance.
I'm 99% sure computers don't make up for it, as there is no knock sensor or lambda probe in the K-Jet system to tell a "puter" what to do with the ignition timing. The ECU in the 16v I think just adjusts the ignition slightly depending on engine temperature.

With the lower octane fuels you basically get...
a) more chance of engine pinking, especially if you cane it.
b) Less miles per gallon.
With my 16v, the cost per mile was only a few pence different between normal unleaded and v-power, as you get more miles per tank with the higher octane fuel.
As for comparisons, I get 350 miles out of a brimmed tank of v-power, prob up to 380 if I risked draining the tank completely.