Perhaps they're trying to change the basis and image of the car. Instead of just straight line performance they are using IRS to create a well balanced, yet powerful car that can take the turns and run at the strip at the same time, albeit not as good as the previous generation F-bodies for straight line performance. I think this a great opportunity that GM has looked into the "total package", so to speak. IRS is a more flexible and versatile rear suspension platform to work with, however; as you stated, it is no the best for drag.