I think I'd go after Forte if I were you...You seem to have some good bench depth which allows you to pull the trigger on a move like this. I think Forte as your RB2 is an upgrade and Mathews or Wallace as a flex each week is very solid. I like it for you.
12 Team League (QB/2 RB/3 WR/ TE/ K/ D)
QB: Rivers, Cutler
RB: Turner, Benson, Torain
WR: Calvin Johnson, Hakeem Nicks, Greg Jennings, Vincent Jackson, Lee Evans
TE: Chris Cooley, Kellen Winslow
K: Garett Hartley