Hmm, let's see...4/5 of the members of Dream Theater were scholarship winners to the Berkley School of Music, the keyboardist went the Julliard Music academy at the ripe old age of 11...
Yes! Coming up with a rap on your feet is comparable.
And get things straight, most of it is NOT coherent, at least not in Rap music today. In the early days they tried to synchronize the phrases, and actually did dance moves. Now they just shout over each other, and shrug their shoulders, while looking pissed off.