I never pit these 2 cars against each other, I am curious now.
I did the last seasonal at Monza with Ferrari 458 Italia, just bought another one, no oil change, straight to the track at Monza with chicane, grip real, sports soft tires, no ABS, no other aids, 10/7 BB, 1:54.452.
Mercedes Benz SLS AMG bought brand new, no oil change, same set up, SS tires, no ABS, 10/7 BB, got a time of 1:55.038.
The 458 is lighter, wilder and agile in corners, it can reach 173 mph at main straight and 165 mph at the back straight. 458 has much less weight at 1485 kg, whereas SLS has 1620 kg, the weight alone favors the 458.
The SLS AMG on the other hand, struggling at acceleration and straight line speed, it barely reach 170 mph at main straight and getting only 161 mph. This is due the rear wing providing down force, which also makes the car more stable in cornering and less prone to oversteer. If only the SLS is lighter.
For added fun, I bought another brand new Mclaren Mp4-12C, no oil change, SS tires, no ABS or other aids, 10/7 BB, this one surprisingly not much faster than 458 Italia, clocking in at 1:54.092.. More laps would get into 1:53.xxx easily.
The car has a lot more power and even less weight than both SLS and 458, reaching 180 mph at main straight and 168 mph at the back straight. Looks like sheer power managed to compensate for less stable handling in corners compared to other two.