Gabe Gross versus righties: 125 plate appearances .269/.368/.398
Gabe Kapler versus lefties: 72 plate appearances .260/.333/.520
Both have gotten at-bats against their same-handed brethren as well. Gross has about an extra 20 at-bats, but otherwise the pair has combined to be something like a ~.250/.350/.450 right fielder with above average defense to the tune of about seven runs.
Even if Joyce were Werth from day one, it's basically a zero sum move at this point -- unless you make the point that having Gross/Kapler as a fourth outfielder everyday instead of every other day provides an upgrade.