Have you ever had two adult passengers in your car? Can you feel the difference the weight makes long before you're driving like a maniac? QED.
Yes, but that weight isn't evenly distributed - a Cab with the roof down over the rear wheels is actually pretty good for grip, where as a passenger in the front seat isn't. I've driven both on track, and own the Cab. There's not much in it in real world driving on the road, there is obviously on track. Everyone will have their opinions, mine are based on owning both and having tracked both, others can base their opinions on whatever facts have informed them. Not disputing your thoughts on this one DRZ, but lets face it, on this forum everyone is quick to jump in on stuff because they read it once somewhere, so I'm just stating my thoughts and where my opinions are based.
To clarify, I also own and track a 911. It's better on track than the M4, and better on the road than the M4 - so it's not like I'm saying the M4 is the pinnacle of a track car, it isn't. But I do know a thing or two about handling and fast road use / track use.