For what it is worth..................and this is only my opinion
The Porsche is by far the easiest of the GT's to drive and don't be deceived by its apparent lack of top speed, because it corners like it is on rails!! It will only lose out at the major speed circuits, and not by as much as you might think. The default setup is already pretty good, maybe adjust the steering ratio to your liking and the final drive ratio to suit the circuit. In time you can tweak things to suit your preferences.
The 2 Chapperals are similar and the 2F is marginally faster because the wing gives a slight advantage. Probably faster than anything else in a straight line, challenged only by the Mk4 Ford. The gearbox takes some getting used to, but the sheer grunt of that massive engine more than compensates.
The Ferrari is a nice drive and with a few setup tweaks is a very smooth predictable drive and quite fast enough to compete with all of the other cars.
The Mk1 Ford needs some serious persuading to compete, but iy can be made to go surprisingly quickly
The Mirage is basically a lightweight Mk1, but is quicker because it is lighter.
The Mk2 Ford took me a long time to tame but has a good turn of speed, but requires some respect in the cornering dept.
The Mk4 is a real beast and it took me time to adjust to the handling that is closer to that of a supertanker, and with a monstrous engine that just keeps on delivering more and more speed. Try it down the Mulsanne Straight along with the 2 Chapps. and try not to dribble.
I'm sure that others will have their own opinions about the cars and can probably offer some setups.