Very cool to get your perspective on the question!Okay - I had to settle this, or at least give you all more to chew on. Anyone can do this test in Forza Motorsport (be nice, this is my livelihood!).
So... I am not a professional racer, I am a professional racing game maker. My laps probably favor the street car, not just the fact that I have some practice but the raw speed takes less talent to maximize. With the same race compound on both cars I am 4 seconds faster in the stock road car. No tuning on either car. The Street car is hitting 207 for me on the Mulsanne and that gains a lot of time. The GTLM is carrying tons of speed without much drama thru the curves.
But, as I said I am not the fastest so asked some of the fastest in the community -
2017 Ford GT (Street w/slicks) R817: 4.00.348 (333kph top speed)
2016 Ford GT LM (stock/no tune) R845: 4.02.710 (289kph top speed) = +02.362s
Race car is insane in the Porsche Curves but stock GT is a monster on the straights!
He agrees that the tuned race car can go faster but thinks that is true of the street car too. Obviously the race teams spend enormous amounts of energy doing just that.