I think a speed of 25' for medium armour is not a bad idea. The only issue is how does it scale up or down for small/large characters?
Random thought: it'd be neat to change the speed limit by armour to something that determined movement speed based on the Strength of the character and the weight of the armour, and perhaps something based on their overall load and Constitution to determine how long they could move at full speed.
That way, perhaps a character with super Strength (24?) could move 30' per round in full plate. Or on a more moderate scale, perhaps a character with Strength 16+ could move 30' in a breastplate, while a character with Strength 14+ could move 30' in something lighter, like a chain shirt or chainmail, and if they put on a breastplate they'd be reduced to 20'.