The stat I used for this was "offensive winning percentage" or OWP. It is a Bill James stat that says what a team's winning percentage would be if it had a lineup of 9 identical players who all hit alike and they gave up an average number of runs. Since I got the data from the Lee Sinins Complete Baseball Encyclopedia, it is park adjusted. I included all players who had at least 300 plate appearances in both 2007 and 2008. The top 25 are below:
I was surprised to see so many players aged 30 or more (11) plus 5 more aged 29. I thought that it would be younger players who improved. Maybe the older guys fluctuate alot more so bigger improvements are possible. But the leader and the #6 guy were both 36. One guy was even 38. The number of players aged 33 or more equalled the number aged 24 or less. The next table shows how much these guys improved in more conventional stats.