You guys are right. Cars with a solid axle will perform as well on a track as a car with IRS. Look, I can post pics of a vehicle exiting a corner as well. Unfortunately, it doesn't show how fast it's going, or how slow it is in relation to the cars around it.
Bottom line, with all things being equal: power, torque, weight, engine layout, and weight, a car with IRS will put power to the ground more efficiently than a car with a solid rear axle. There's really no denying it.