Bike Fitting

My road bike is currently set up with a layback seatpost and a 90mm stem, what changes in handling and power input would I see from fitting an inline post and a 110/120mm stem? shifting my weight further forward on the bike, does it make any difference?

Depends on the rest of the frame geometry. Stem length is not to be viewed as de-coupled from its height (saddle-handlebar drop), head-tube angle, seat-tube angle, top-tube length, saddle position (the offset behind the bottom bracked as measured by dropping a plumb line from the saddle) etc. Is your saddle pushed back? Foward? All these "variables" have an impact.. Shifting weight? Hardly so... Stem length has such a minor effect on the weight distribution that you can literally forget about that factor in your models.. What stem length effects, however, is steering! That is why road bicycles have tended to evolve with comparatively longer stems (less twitchy)... 90mm stem? One often talks about stem length in comparison to top-tube length (but again, top-tube length can't be de-coupled from saddle height and position, viz. also seat-tube angle). .. and all of it can't be de-coupled, again, for handlebar height.. and even handlebar geometry (drop, reach etc.)..