[1thump]

How do you think it would run with just the header?

[Otto_Ing]

My friendly neighbours would become less friendly and i would get a visit from my friends the police.

[High On Octane]

How do you think it would run with just the header?

Not nearly as good as with a muffler.

[ace.cafe]

If you don't want a muffler, then use a megaphone.
The engine won't like it with just an open header.
And it will probably need a Power Commander, even with a megaphone. When free-flowing the exhaust, you always need to make the fuel richer.

[hyperhad]

Don't forget some ear protection as well!

[NilsyfromtheNock]

So is there a limit to what the stock O2 sensor can do to richen the mix?

[ace.cafe]

So is there a limit to what the stock O2 sensor can do to richen the mix?

There definitely are limits.
Each bike seems to have its own personality in this regard to accept free-flowing exhaust, and some seem to accept it better than others. But at some point, a Power Commander type device will be needed, and it might be best to just get one at the time of getting the free-flow exhaust, to eliminate the question mark, and allow you to adjust the map as needed.

Just as it was known with the old Iron Barrel Bullets that the factory jetted the carb much leaner than optimal with the stock showroom bikes to pass emission testing, I would say that they are doing the same thing with the fuel injection for emission purposes. And unless you have something like a Power Commander device to adjust it, then you are stuck the way it came from the factory.