I think there was about 2 seconds between the Viper and the 550 for me and I definitely tried harder with the Viper.
No doubt you can go faster than the laptimes I did. I used 60L and I drove them as if I was doing a race stint.
Viper is the most fun to drive in my opinion.
Lambo was hardest to drive, at least around here. It's a lottery over the kerbs which slows it down considerably in Variante Alta. I also couldn't get temps in the front tires like a I could with the other cars.
The lap with the 575 I did is not representible. It handled like a dog and I got bored. Pretty sure it can do a 42.5 or .6 just like the Lambo and Lister, if you have more patience than I do.
Saleen is at least half a second faster than those. I don't think I had a good run through Variante Alta with it. It also seems to kill its tires the fastest of the bunch.
550 is another second faster than that.
A track that is less dependent on one corner with massive kerbs would be a good idea for the next step in BOP testing.
Also don't just start slinging lead around. AC also has the option to change the intake restrictor. This mostly reduces power at the top of the power curve (where the engine needs the most air). This means that it doesn't really change handling, braking, fuel use (a bit, but less than 50kg lead and in the opposite direction) and so on.
I agree that simply taking the 550 out would be the easiest solution but if we can balance them then that's the preferred option. Maybe not all the way down to the Viper, but I'll just drive that then.
Oh and btw, the diveplane option for the 550 is only visual.