Rear tyre sizing

I would suspect some additional understeer will be induced with such a wide rear tire. It IS a popular thing for some 355 owners to do, as I have seen discussions of it here...but if you don't increase the front a bit, I would think the balance might be upset. But I am not an expert on such things.

 

Rolling radius:
a 295/35-ZR18 has a rolling radius of ((295*0.35*2)/25.4+18) = 26.12
a 265/40-ZR18 has a rolling radius of ((265*0.40*2)/25.4+18) = 26.34

No damage done, here; its close enough.

If this added width out back causes understeer, then you can raise the rear by 1.0-to-1.5* turns on the spring perchs and rebalance the understeer/oversteer relationship.

(*) assuming the car was in complete balance before the wider rears were put on.

There is such a thing as too much tire (too much traction). When a nimble car has too much tire, instead of being nimble and able to dance along the road, the added traction makes it a bit ponderous and heavy (feeling). Corvettes (and Vipers), in my opinion, have this ponderousness that Ferraris do not. It is an interesting trade off because all of the number you see in the road test magazines (typically) go up, but instead of dancing with Ginger Rodgers, you are dancing with Mohamad Ali (in his prime).

One of the characteristics that makes Ferraris fun to drive is the feedback the driver gets through the wheel (and ears). A lith car will go from pure grip, into a gentle slip, into agentle slide, and finally spin. A car with too much tire will grip, grip some more, grip even more, then spin out. Unless you have Herculean sensors in your car control tolol box, you won't see the spin comming. Thus an overtired car is closer to the edge and easier to go over it; while the properly balanced car is giving massive feedback about the perils that await.

In the tires in question, we are only talking about the last 10% of traction. So the above paragraph is overstating the reality of the situation by a tad.

 

The F355 sits at a rediculously* low 4.2 inches of ground clearance from the factory--for example, this is 2 full inches lower than a Corvette. It does not need to be any lower. Indeed, I often scrape the undertray in daily driving, and occasionally bonk the nose. I can't imagine living with normal road surfaces and sitting at 3.2 inches of ground clearance.

Also note, when the car is lowered 1" both front and rear, the rear roll center goes down by 1.65" while the front roll center goes down by 1.12" {pure suspension geometry}. This induces understeer.

(*) my opinion