Fuel won't make much of a difference really, from what I can gather you only gain the benefits of higher octane fuel if you're actually mapped specifically to use it. And even if you could gain a slight power advantage, I would imagine his setup would give him an advantage in the corners anyway, so I'd say you're probably stuck as you are.
Having said that, it's mostly about the driver anyway
Oh, and another thing.. if he wins, you say "well you had the faster car, so what?". And if you win, you say "omg you lost even tho you had the faster car! LOSER!!". So you got nowt to lose really