I'm running a 5" Fox Vanilla on Marin Trail pro which is similar. Yes it slackens the head tube into a more downhill like stance so it works much better going down than up.Climbing is harder but I compensated with a longer stem and moving my seat up on the seatpost so its working out fine. Going another inch I would not do. Also if you are planning to put on a double crown fork you better check with Marin, I'm not sure if the headtube is burly enough to handle the stresses of a double crown fork.