my 01 mxz 600 already has 280 main jets in it (stock), and i want to jet down because its running rich as ####, as always from the factory. *so if i go to 270 or 260 will this lean it out?

yes it will lean it out,the lower the number the leaner you get.

moved to Ski-Doo

woops i found out that its got 500 main jets, not 280's

I thought 280's sounded lean for flat slides ,but you never know if a guy switched to round slides.

In stock trim my mxz 800 came with 500 main jets, would the 600 really be running the same main jet as my 800?

yes they will, bcuz there the same carb.