Weight will also be a massive factor. Increasing fuel rates would obviously open the opportunity to more power from the combustion, but you would have to carry more fuel to accommodate it. So in simple terms twice the fuel would increase the race lap time from circa 5 seconds above quali time to 10 seconds above quali time. Even if you allow for a massive performance gains with the additional fuel allowance, its unlikely to fully neutralise the deficit incurred by the additional rate.
Now that you have raised the question, it would be interesting for one of the engineers within the sport to indicate (after the relevant trade offs) what the optimal fuel allowance would be for a GP, offsetting weight, race pace, increased tyre wear and fuel saving etc.
That's certainly a dimension to this season I hadn't considered. Last year, teams regularly raced with less than the 100 kg allowance on board. But with more downforce and more resilient tyres, a greater percentage of the race will be run at something close to "maximum attack". Is saving fuel rather than protecting tyres going to become a bigger factor in determining race pace?

) they were with BP/Castrol who are the same supplier as the works Renault team. But now they have Renualt power themselves they have Petrobras who also sponsor williams who behind the scenes use Petronas with their Mercedes engine.