Do a thread search for this one. I think there's a couple ppl who Modded the airbox and added a aftermarket filter only. Dont think you would see too much gain with filter ONLY. You would have to loosen up the restriction into the airbox either by removing the lid, removing snorkel, or modding the airbox with holes or "hole inserts" though. Do a search though, you'll find a lot of info on most of this.
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.
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