I would think that a good caching solution on the server for RSS requests could perform decently even under DDoS scenarios... though, this is why people used other services in front of their RSS feeds, so that they were better cached. Most blogs aren't generation more than a couple new articles a day, so caching 15-120 minutes wouldn't be an issue for most use cases.
And if your web application allows queries that produce RSS feeds that could still result in a really bad L7 attack if you simply were to ignore all feeds. No caching + randomized queries on a small site would knock it offline in no time.