Let's say a Masterwork Sledge (treated as an improvised Earthbreaker); would someone wielding it gain the +1 Enhancement bonus that a Masterwork Earthbreaker would grant?
no, it's not a weapon
No, the Masterwork quality only gives you the bonus specifically stated in it's description. Weapons get +1 to hit, armor reduces ACP by 1, tools give a +2 bonus to the relevant skill.
Also the first line of the Masterwork Tool description from UE "This tool is perfect for its intended job."