Just do what some others do: either get in your car and record the segment or use digitalEPO to modify your file before uploading to Strava.
Joking aside I wouldn't say you'd need to hit 50MPH. Just hit 43MPH and you should hold the title for a while.
It's entirely do-able especially for us Clydes believe it or not! In downhill segments I regularly beat out my sister and brother-in-law since I have more "forward momentum". I've got almost 80lbs on my brother-in-law, and although on the straights he whips my butt six ways from Sunday on his carbon Cervelo bike, time and time again I blast past him on the descents. My fastest downhill run near Harvard, MA (the town, not the college) was around 44MPH and that was when I had Specialized Armadillo tires on my bike. I'm willing to bet I could hit at least 45MPH on the same hill I did 44MPH at with the gatorskins I have on the bike now.