The speed that you ride at is dependent upon so many factors... it's really hard to say whether 50km/hr should or should not be possible for you.
What actually matters is whether or not you outsprint your rivals at the end of a race. I've won a bunch sprint at about 25 - 30 km/hr, but it was up a biggish hill. On the other hand i've been at the back at ~ 70 km/hr on a downhill sprint.
Thus the terrain that you ride on, along with any local environmental conditions will have a mjor effect on your speed. This is why you'll often see people quote a peak power for sprinting ability (e.g., 800 - 1200 W) and often this will be quoted as a power to mass ratio, to put things into perspective.
However, if you are struggling in the sprints in races, then some dedicated sprint training is what you'll need to include in your training (e.g., five x 10-secs, starting from [almost] stationary, in a very low gear ~ 42 x 19, whilst remaining seated, with a *LONG* recovery period between each interval > 20-mins). On top of this you'll also need to practive techniques, such as holding a wheel at high speed, and coming off a wheel to make a dash for the line, etc. Sprint technique is crucial.
Ric