My bike came with a 110mm stem but when I swapped to campy, the shifters are longer than shimano so it makes my stretch out more, just a little but makes a difference (im sure campy users can chime in).

How much of a difference will switching from a 110 to 100 or even 90 cause?

I thought about 100 but not 90 because I think 90 will be too short but I do not have stem experience.

I often place my hands right before the shifters but when I get out of the saddle I feel normal on the shifters.

BTW, my bike has a Look HSC 4 fork on it, it uses a 31.8 correct?