a FWW will make the engine slower and less willing to acelaration, positive or negative.
they may make the bike get more traction easier since the engine will be slower to rev. yep, on a good traction terrain and high gear you may loose some speed. the bike will have less engine brake as well.
actually, a FWW may loose more traction than the conventional setup. such as when leave a corner with lots of throttle and clutch. the added inertia on the moment of disengage the clutch may brake the traction.
hope it makes sense...