Polling for time sensitive things sucks. But for RSS, why not just poll once a day? Even if you're polling once an hour, a 304 response is, what, a few hundred bytes?
I just checked my own server logs, and HTTP 200 responses were 12.7% of the total requests against my rss.xml. Which is suboptimal I guess (I haven't made a single post this year) but isn't outrageously terrible.