Taking into account the 2 finalists in the last 10 HR derby's (20 players), there has been a 6.2% drop in offensive performance (wRC+) in their 2nd half. Compare that to the league average performance drop in the 2nd half during those 10 years (4.1%), that's an additional drop-off of 2.1%. Nothing significant. It's worth noting that Ryan Howard had a mammoth 2nd half in 2006 which is skewing the data a little, and Bautista was injured soon after the event in 2012.
Raw:
Player (year) 1st half 2nd half Diff
Frazier (2014) 138 97 -29.8%
Cespedes (2014) 109 110 0.7%
Harper (2013) 146 129 -12.4%
Cespedes (2013) 95 112 12.4%
Fielder (2012) 135 173 27.6%
Bautista (2012) 141 80 -44.4%
Cano (2011) 129 138 6.5%
Gonzalez (2011) 168 137 -22.6%
Ortiz (2010) 145 123 -16.0%
Ramirez (2010) 129 123 -4.4%
Fielder (2009) 174 146 -20.4%
Cruz (2009) 115 111 -2.9%
Morneau (2008) 138 114 -17.5%
Hamilton (2008) 136 125 -8.0%
Guerraro (2007) 144 141 -2.2%
Rios (2007) 124 120 -2.9%
Howard (2006) 125 202 56.0%
Wright (2006) 145 112 -24.0%
Rodriguez (2005) 165 183 13.1%
Abreu (2005) 148 103 -32.7%
Total/Avg 137.5 129.0 -6.2%
League (2005-2014) 98 94 -4.1%