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This is remarkable. 
You're saying that he has a slider-speed bat based solely on one statistic - the B-REF definition of 'power' versus 'finesse.'   If he actually had a slider-speed bat and simply could not catch up to a fastball, what would you expect to his pitch type scores to be?   Why has he been eaten alive by sliders?   Secondly, what's the context?   What's league average for these pitcher types, and what does it look like if you use the same BABIP for each sample?   Finally, wouldn't you expect a poor total RV for FBs given the fact that he's been a poor hitter - overall, against everybody - in his career?
Let's take another tack.   Player X has a career OPS vs. 'power' of .699, versus a career OPS vs. 'finesse' of .909.   This gap dwarfs the one you pointed out for Langerhans.    Should Player X sit versus power pitchers?  
The point here is that you haven't proven that this statistic is correlated - at all - with batspeed.   If this were a fatal flaw, results like Langerhans' 2009 would be pretty much impossible.   You're saying that once the league learns to throw him all FB's, he's toast... but it's certainly relevant that he's hitting FB's BETTER than off-speed stuff RIGHT NOW.   He's got 2 HRs on FBs off of *power pitchers* this year.