M4 v ISF / GSF, from what I’ve seen on the road.....in a straight line drag, the M4 is faster - no doubt. The M4 has a major advantage with torque, so if you were up against one, you’ll have to work harder and rev your F - not necessarily a bad thing.
Mix in some wet weather and the advantage goes to the F massively. I also suspect the average driver could push an F more comfortably than an M4.
Like I said - these are my observations after seeing several F cars following an M4.