The thing is, if a guy's pitch velocity declines significantly, you usually find out about it first by him getting hammered. There's your accuracy test right there.
I think if you're using such data for serious purposes, you're going to go a little further to authenticate that it's reliable. That would include consulting game scouts etc.
We all remember Pedro's infamous relief stint in 2004 ALCS Game 7. After giving up the 2 runs Pedro was throwing smoke to get the last 2 outs of the inning. The radar gun had him throwing high 90's on a few of those pitches. I've got the DVD to prove it, as I'm sure a lot of us do.
Hanley's final fWAR for 2015 is -1.8. An average player's fWAR is about +2. So Hanley cost us roughly 4 games compared to an average player. And he only played about 2/3 of the season.
Seriously, you haven't noticed a huge difference in our outfield defense over the last month?
Hanley's negative impact is quantified in his defensive ratings, which I believe are the worst of any outfielder in baseball this year.
I think the improvement in the outfield defense has also been a sizable factor. His absence from the lineup hasn't hurt the offense either. Let's face it, the guy was horrible. It's not easy to pull off a WAR of negative 1.8.
Hanley was brutal this year except for his power surge in April. How much of it was s*** luck and how much was his own doing, who knows. But I too would be happy to see him go. Once he stopped playing we started playing like a good ball team.