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A few points at random:
1-- Traction vs. friction. Physics-book friction is not tire traction. They are related, but not the same. For one thing, the "coefficient of friction" (mu) isn't actually a "coefficient", in that it changes depending on lots of things. For another, tires also have mechanical grip that does not depend on friction. Bits of the tire actually interlock with the pavement, providing more grip than simple friction. (That's how cars can use their tires to accelerate or to corner at greater than 1 g.)
2-- Mass versus cornering. theunchosen was quite right when he pointed out that a lighter-weight vehicle will have higher cornering speed (given equal traction) than a heavier one, because the heavier one has to have more force applied to it to change its direction. Now, traction will not be equal without lots of "fudge factors" (like extra downforce on the lighter car, or grippier tires, or whatever), but the lighter car can and often does wind up with a higher maximum cornering speed.
3-- Tire size versus tire traction. Once again, tires do not work like textbook physics. Real world experimentation tells us that wider tires do indeed provide more grip than narrower ones (all other things being equal). I suspect at least some of that is also tied into the mechanical grip between the rubber and the pavement, and there may be other factors as well. But we know from real life testing that a car with wider tires (up to a point!) will have a higher cornering speed than one with narrower tires.
4-- The GT-R is not meant to be all things to all people. It is meant to be a nice comfy street car, with decent passenger space and cargo space and "all the toys", that can go like stink on the Big Track and still works pretty well at the drag-strip. You can call it "not an engineer's car" if you like, but it took a lot of quality engineering to package all of that together. Unfortunately, it does have some problems that have gotten Nissan bad PR.
I don't particularly care for the GT-R. But I do think it's a good piece of engineering. The market that it was built for does not include me; I am not that interested in it except in the abstract. I wish it had been designed to meet different goals (e.g., much lighter weight), but even then I probably wouldn't be interested in it nowadays.
And I don't think turboshaft engines are the be-all end-all. In fact, it sounds a lot like they need to be made much more efficient at producing small amounts of power, and at dealing with varying loads, before they are truly viable in a car. They have possibilities, but are not quite "there" yet.
-soD
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