[Ecky]

Some things to consider:

Wider tires will hurt aerodynamics, especially if they begin to protrude from your wheel wells.

Wider tires in theory have lower rolling resistance, all else being equal, because they have higher load ratings and at the same pressure your tire is deforming less over road irregularities. HOWEVER the flip side to this is that wider tires do not actually have more grip at the same tire pressures either - because your contact patch is essentially the same. You have to drop tire pressure to get any more grip out of a wider set of rubber, because otherwise your contact patch stays roughly the same - it's a function of vehicle weight and air pressure.

(this ignores sidewall stiffness as a factor and assumes rubber is perfectly pliable, which it is not, but the above is *mostly* true in practice)

If you go wider and leave the pressure the same, your contact patch (and thus overall grip) will be about the same, but it will be shaped differently. The contact patch will be wider, and you'll get slightly improved lateral grip at the expense of braking and accelerating grip.

Although your rims will be lighter if you go with the the 265's, your wheel's overall rotational inertia will still likely go up, resulting in doubly reduced accelerating and braking performance. This is because although the rim in the center is lighter, all else being equal the part of the wheel that's farthest away from center (the tread), the part that affects rotational inertia the most, will be wider and heavier. Tire weight matters a lot more than rim weight.

What impacts grip the most is tire compound. The largest reason I can see to change tire size is to have access to a different set of tire compounds. If you want the fastest car possible, with the best acceleration, braking and handling, you'd get the smallest rims that still fit over your brakes and go with a narrow tire with a very sticky tread compound.

Why do supercars have super wide tires, then, if not for grip? It's actually heat dissipation and tire wear with the super sticky compounds their tires need. Wider tires can dissipate heat better, and wider tires at the same pressure will last longer, because you have more tread without increasing contact patch. Everything else (acceleration, braking, handling) gets worse.