With no other mods? umm.. maybe a 135? I doubt you'll notice much of a difference with just a filter and that grinding you want to do.
The stock jet is a 130. The Z400 seems to be mainly held back by poor breathing due to the sound baffle at the end of the snorkel & the pea shooter on the spark arrestor. After that you'll have to cut a hole in the muffler (the wall directly behind the spark arrestor) and cut up your airbox lid.