Sure, 25... not sure where I got 17. It's approximately the same level of meaninglessness since it amounts to a little less than a week of baseball.
I suspect if you look at players with a significant number of playoff appearances in their career, you'll find that their career regular season numbers are far more predictive of how they perform going forward in the postseason than the first 5-10 games of their postseason career.
I don't understand how you're not projecting forward when you're factoring it into the size of contract he should get. The contract you give a player should be based entirely on how you project them to perform for you going forward.