soulfly,

does that mean if my fueling will be a lot more richer if i remove the cat?
i thought it's gonna be tad leaner due to more airflow meh?

so if the manufacturer runs it a li'l leaner, that means you're screwed.

they don't necessarily run rich - by all means, do a checkup on your spark plugs to ensure that theory before saying so. it's the cheapest method available.

and a/f ratio doesn't give good fuel mileage without proper driving technique - if that's what you're after.