Yea they were...the Mariners took off after ARod was dealt. They never won more than 91 games in a season while he was there (and they only did that once, in his final season, 2000), and as soon as he left, they won 116, 93, 93...granted it tailed off to where they're at now, but the immediate impact after his departure is hard to argue with.
EDIT: I just re-read what I wrote...I said "before they had him" rather than "without him", my bad. Still though, since he became a superstar, the two teams that got him were better before, and when he was a hotshot young ballplayer, the team that dealt him improved after he departed.