Trying to find a player similar to Lance Berkman isn't easy. He's fairly low on PECOTA's sim score, and that's probably because Lance is a rather unique player. Finally, I stumbled across Berkman's Sim Score on Baseball Reference and I decided to look at his "Through Age 32" comparables to use as a loosely predictive tool.
The graph below are Larry Walker's (Lance's highest comparable up to age 32) and Berkman's career WAR and then I extrapolated a little bit further into the future to see what, if anything, we can expect from Berkman through age 36; as he said he was leaning towards playing for another year or two.
I'm not sure how much this tells us about Sir Lance because Lance's career WAR fluctuations reflect his rotation through the defensive spectrum more so than telling fluctuations in his bat. Perhaps the better effort, which I won't attempt now, would be comparing BRAA (batting runs above average) so we can isolate what to expect from Lance at the plate, instead of having his fielding challenges become obscured by measuring WAR. Nonetheless, as Lance settles into 1B, he's career path should tend towards Walker's, who stayed in RF for the majority of his career.
Walker's age 33 dip is the result of a precipitious and analmous drop in his fielding that year. I'm not sure if that was injury related or not, but his bat was solid that year. Hopefully Lance can replicate the batting contribution, but hold on his stellar defense from 2008.