Precisely how does Rammus's movespeed boost (powerball) work?

Rammus goes faster during Powerball. Unlike almost every other ability in the game, the tooltip for Powerball doesn't specify how much of a speed increase Rammus receives. Mobafire, Leaguecraft, and SoloMid don't appear to have specifics available, either.

Does the ability in general increase your movement speed by a percentage, or a flat amount?

Is there any credence to the hearsay I've encountered which indicates that powerball ignores the soft caps on movespeed?

Solution 1:

Rammus' powerball appears to be a % speed increase, reportedly increasing about 10% of his movement speed per second.

This means a Rammus with 5 phantom dancers will have a significantly faster powerball than a Rammus with just boots.

http://leagueoflegends.wikia.com/wiki/Movement_speed

The reason Rammus appears to avoid the soft caps on movement speed is because Powerball compounds his movement speed - it's being reduced by the caps, but it's still a significant enough amount that it does increase, which means the next application of Powerball's "speedup" tick will apply more than the previous, even with the soft cap.

These can combine to push Rammus' speed into egregious territories.