The best way to test this is to replicate it.
Get out there again, pull out the watch at the top of the descent and at the bottom for verification, and then sum the distance and speeds at home afterwards. You’ll get a fair idea of whether you were in the ballpark.
Eye-balling the difference between the curve length and the road length where they separate and estimating the right triangle angle I’d guess the GPS added 53.5=x/cos(15 degrees) ==> x = 53.5*(.965) = 51 mph. I’ve gotten to 50 mph on 6% straight descents, so it’s probably close to real.
Was it fun?
It wasn’t too bad. I looked down and saw something like 41mph, then the real acceleration began. I tucked a bit, but didn’t dare get onto my aero bars. I’ve never crashed on my bike, and I didn’t plan on starting at 50mph.
Pretty impressive wind sensation, to be sure. My previous best was a bit under 49mph (IIRC) during a descent at Timberman that wasn’t nearly as steep, and I was on my aerobars at the time.