Q: Does Full Text RSS Feeds pull in all posts from a blog, or all the content possible for the posts provided in an RSS feed?

The limit on the number of feed items provided in the RSS feed is set by the provider of that particular RSS feed, and we cannot do anything to change that ourselves. 

What the Full Text RSS Feeds add-on does is it provides the full-text functionality to an unlimited number of feed items. In other words, while with the free full-text service you’re limited to the full-text functionality for only 5 feed items, with the premium version the full-text functionality will apply to as many feed items as your feed source provides.

Full Text RSS Feeds, in combination with Feed to Post, does not and cannot import any more feed items than those provided by the RSS feed.