50km/h for a short amount of time is far from unusual for most cyclists. The fastest cyclists can do 90km/h in a burst which requires ~5x more effort than going 50km/h.
Most people with a little training can do ~1200W which is enough to get to those speeds with a good position on a well adjusted bike.
Can you give me an example? I built my own ebike so I'm familiar with the componentry, going something like 40mph (ripping past 30mph) requires a serious battery and a very expensive motor. You can't just modify a normal legal ebike to go that fast, the motor will overheat and the battery won't last 20 minutes on a charge or long in general.
Also, nothing that can go past 28mph is considered an ebike in the US. The moment it goes any faster it's an unplated scooter/motorcycle