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So you're going to stick with your theory despite the fact that the metric you used is not actually measuring pitch velocity, and isn't actually considering pitch type.  If you wanted to prove a theory like this, you absolutely would NOT use the b-ref pitcher type stat - you'd need pitchfx data and you could do a plot with FBs categorized by speed.   Then you could see if Langerhans was far below league average vs. medium/hard FBs, and dominant against slow ones or something like that.   But using the B-ref stat as a proxy simply doesn't work, and it seems like you'd rather cling to the conclusion than examine other hyptheses.   The example in the previous post by the way - the mysterious Player X - is Russell Branyan.   Please explain why you think Branyan has a slider speed bat given his 'disastrous' splits in the same stat.   He makes Langerhans look like Ichiro.   I'm not saying that Branyan has a slider-speed bat, just as I cannot say that Langerhans doesn't.   I'm saying I don't know, and can't tell given fangraphs pitch-type LWs and B-ref.   The one thing I DO know is that you don't know either.  
The reason is that B-Ref's definition of 'power pitcher' is simply someone who gets Ks and BBs.   It isn't trying to measure speed, and thus would give equal credit to Trevor Hoffman and his 85mph heater as it would to Jonathan Broxton and his 97mph FB.   It also wouldn't account for guys who rack up Ks on slow stuff, like Hoffman (or even Lincecum), which is why Branyan does well v. FBs in the fangraphs metric, but is hopeless against 'power pitchers.'   This isn't the right tool for what you're trying to measure.  
If you use the fangraphs pitch type data, yes, you see he's got -7.5 runs on the fastball, which again is not at all surprising given the fact that he's not hit very well over his career and the FB is the most common pitch.   As a comparison, Jose Lopez is at -27.7 runs vs. fastballs.   Clearly a slider-speed bat there.    Yuni Betancourt has roughly twice the PAs of Langerhans, but his career FBv isn't ~-15, it's -36.5.   Erick Aybar's at -9.2 for his career in fewer PAs, but he's an above average hitter this year.   Jimmy Rollins is at -7.0 on the fastball in 2009 ALONE, while Robinson Cano was a stunning -16.4 just for 2008.   By the way, this year?  He's at +15.0.    Again, if you want to see if someone's batspeed is slipping, that's cool, but you'd probably want to plot their LD% or even slash line against different groupings of fastballs.   You could check it against hittracker's batspeed measures too.