I managed to do 45.4 mph down a French hill last year on my trike. It was steep, but very bumpy with a sharp bend half way down. I was determined to exceed 50 mph, but I bottled out when the trike was leaving the ground on the bumps while I was simultaneously leaving the seat. On a smooth road 50 mph would have been a doddle.
I am a rather large gent to say the least, and gravity is very much my friend when going downhill. I don't have much more wind resistance than a thin cyclist, but there's a lot of force pulling me downhill. I was out with a fit friend on a road bike and on our local downhills he was forced to pedal to keep up with me on the trike, while I was serenely coasting. I shall draw a veil over my uphill performance compared to his though.