Every player sans one had a positive fWAR total after being given a contract extension. That's good! The key to arbitration contracts is beating market value. For a large portion of these contracts, one Win Above Replacement varied in value from $6 million to $10 million. That's where it gets dicey. Was there surplus value provided by the player after signing his contract?
There are some obvious good contracts there. Yadier Molina, Adam Wainwright, Matt Carpenter, and Kolten Wong all provided great seasons after their extensions. Molina signed him after showing some offensive promise in 2007 (.275/.340/.368 slash line). Molina's defense was also starting to shine. During his contract, Yadi made it to three All-Star Games, received three Gold Glove Awards, and he even received MVP votes in 2009, not to mention a World Series victory in 2011.
Adam Wainwright signed his extension after cementing himself as a true starting pitcher in 2007. He started thirty-two games the year before his extension, and he only improved from there. During his extension, Waino went to one All-Star game (Tommy John held him out of the 2011 season), he placed third in Cy Young voting in 2009 and second in 2010, and his ERA was never higher than 3.20 during his contract.
The recently returned Matt Carpenter provided plenty of excess value during his arbitration extension. Carp received one of the longest extensions, and he provided the most value of anyone on this list. He made it to All-Star games during his contract, and he received MVP votes in 2015 and 2018. Carp led the league in doubles in 2015, and he was 30th in all of baseball in fWAR during the length of his contract.
Kolten Wong is the final "good" player to have received an extension during his arbitration years. Wong received a contract extension soon after his 25th birthday in 2016. Before his extension, he placed third in Rookie of the Year voting, but he was otherwise steady on both sides of the ball. His defense was his primary benefit, but he could hold his own on the basepaths and in the batter's box. His 2016 season was underwhelming (1.3 fWAR), but Wong would go on to receive two Gold Gloves and even see some down-ballot MVP votes in 2019. Overall, he accumulated 11.2 fWAR throughout the course of his contract.