Well, it sounds good on the surface but.............
If you have 500 affiliates using the same rss feed, there's no way you can have 500 differently worded rss feeds.
The best thing to do in my opinion, in addition to what you're already doing, is have each affiliate have completely different content.
if you have 500 affiliates using the feed, make sure you have 500 different pieces of content, each affiliate is assigned a different content feed for the day/week/month of the update.
hopefully makes sense..
heh
So no affiliate will ever have the same content on their site on the same day. But google doesnt exactly spider everyones site on the same day, so adding in your piece helps..... GL