Hi there, welcome to iboats!
Are you sure you received the same gear ratio?
Are the propellers original, new or repaired?
Was it a complete new stern drive or just a new upper or lower unit housing?
Due you feel the sluggish is gear related or engine related?
Also for now, we'll be moving your topic from the Mercury O/B forum down to the Mercruiser forum for you unless it's a prop issue.
Good luck.