From an engineering standpoint, going from a 37# flywheel to a 20# flywheel represents a reduction in the moment of inertia (MOI) of approximately 27% for the entire flywheel/clutch assembly. That sounds like a lot and most drivers will notice SUBTLE differences in performance. I say subtle differences because I've driven 30#, 20#, and 14# stock diameter flywheels and I've driven 7-1/4" button clutches (no flywheel) that weigh 15# for the entire clutch assembly. Until you get into the small diameter, lightweight racing stuff, the differences in drivabilty are very minor. Compared to the 37# wheel, the 7-1/4" assembly represents almost a 1000% reduction in MOI. The Cobra is a light enough car that I drove the button clutch on the street for a year. Yes, I had to make adjustments to driving technique, but that was not a major issue. What my experience with clutches and flywheels has taught me is that from a drivability standpoint, the Cobra is light enough to use any clutch/flywheel. But, from a durability standpoint, a clutch that is light enough to make a significant difference in performance is not able to hold up to heat (unless it's carbon), so the lifespan is very short on the street. That said, IMHO there is no such thing as a street-style flywheel/clutch that is too light for a Cobra. If I was buying new for the street, I go with aluminum. If I was buying new and I wanted to spend $6500, I'd buy a carbon multi-disc clutch and have durability and performance. But, if I already had a steel flywheel I'd use it... the performance and drivabilty differences in the stock-style stuff are just too minor to me.
Ultimate street clutch:
http://www.lgmotorsports.com/catalog...c7f9610b5be9df