I second what MotorGeek said. my stock is LT 250r lined up next to our 03 z400 is a blow out. I don't feel that my 250r would be much of a challenge for even a stock banshee. Any 450, or a big bore bike like a DS650 would be better.
As for changing out carbs on a LTZ400 I wouldn't, the stock carb is great on a stock to med built z400. plus I don't think you would gain much by changing it.