Check out Justin Upton's home road splits the last 3 years.
Home - .282/.371/.436/.807
Road - .264/.341/.449/.790
Home - .333/.411/.622/1.033
Road - .246/.328/.439/.767
Home - .313/.390/.534/.924
Road - .252/.326/.344/.670
He's leaving one of the most hitter friendly parks in Arizona and going to Turner Field which is a much more difficult place to hit HR's.
There is some white noise in players H/A spilts. As a whole, players have an OPS about 40 points higher at home. Upton's H/A splits for 2009 & 2010 fall with that norm, Last year he battled a thumb injury all season which could easily explain why we wasn't able to drive the ball (decrease in XBH%,HR% and HR/FB) so you end up with 2011 being the outlier.
Any player moving from a hitters park to pitchers park is cause for concern but I think in Upton's case people are only looking at the surface of what the stats tell us.