Well the new M4 (coupe, old coupe was an M3) did Laguna Seca in 1:39.69, ZL1 1:39.18, GT500 1:38.70, Z/28 1:38.82. It appears the old M3 (2007) ran 1:42.96. The Mustang Boss 302 LS did 1:39.50 and the non LS did it in 1:41.06
LINK
So if the 2015 Mustang GT is faster than the Boss 302 LS (which I find highly suspect) then it would be faster than the new M4.
At VIR the fastest recorded lap time for the following Cars is as follows:
ZL1 2:52.38
1LE 2:58.34
SS 3:09.50 (this was a 2010 Camaro on the FE3 suspension, before GM revised all Camaro's to ride on FE4 in 2012)
GT500 2:58.48
Boss 302 LS 3:02.80
Mustang GT 3:08.60
BMW M3 (2007) 3:05.40
LINK
The VIR laps are a little wonky, having different drivers changes that quite a bit. 5 seconds in the ZL1 from the test engineer vs C&D. Also, I didn't really like the Boss 302 test vs the 1LE at that track because they used a chevy test driver in the 1LE vs an editor in the Boss 302 from what I read.
Also, the GT500 number you posted was a 2010, the 2013 with the car and driver editor was actually 2 seconds slower.
I wish we had another unbiased named driver for car to car. Randy is a good driver but some of the times he has posted seem really weird.. The F12 berlinetta according to Ferrari is the second fastest car they have ever built, only behind FLF but it's only .7 tenths quicker than the heavier, less track focused, less powerful, less tire and heavier GT500. The Berlinetta is usually quicker than the MP4-12C but in this case it's 4 seconds slower.