The RSS feed should go out immediately. Only the RSS widget in the sidebar is cached, because everything in the sidebar is cached. How long till people see it on your site? I have no idea; that completely depends on how you've set up your site and I'm afraid we here won't be able to help with that; we're WP.com focused, not independent HTML focused, so we don't actually use that feature generally.
And yes, I mean your pagerank and the ability of people to find your stuff on Google. Google looks at duplicate content as "gaming the system" and downgrades you. The band Nine Inch Nails, for instance, posts exactly the same content (comments and all) to Blogspot and WordPress.com and their blogs kind of suck for pagerank, despite the fact that the band is famous worldwide and gets tons of hits on both blogs.