AC's stage I seems to be the same as a Litcho stage II (i.e panel filters + exhaust + remap)
I would expect that they are much a muchness and to be honest won't feel a great deal different to the Litcho stage I. With a full exhaust vs a ypipe only i believe you can a run a little more boost but the main difference is the sound change you get by switching to the full exhaust.
If you want to notice a big change i would suggest going to a Litcho stage IV + downpipes (i.e exhaust, intakes, injectors).
Sold: White R35 2010 Black Edition
Current: R32 - Nur engined - KH2 - TE37s