It won't harm, but like I said, it's all in the prep if you get it coated.

If it's stainless already, I would do a test spot, like I said, with some oil and 0000 steel wool, and you'd be surprised by the surface grime you remove, which may show the exhaust looks better than you think.

I would go the least invasive/expensive route first if it were me to see, and yes, 409 stainless will never be as pretty over time as 304SS.

I lived in the South and went with stainless anyway; I know aluminized 'is fine' for my climate, but it doesn't make sense not to spend the extra $150 for stainless.