I finally feel good enough about the refactoring work to post a dev build. The save file formats have changed in this build, so if you download this - make sure to backup your current save file first!. Here's what's new:

Editable settings are saved in a file called btfSettings.ini, and this file lives in the same directory as the application. It's an ini (key=value) file - most of of the value types are pretty obvious
non-editable settings (window positions, etc) are saved in a file called btfWinSettings.btf. This is a binary file, and it will go into your save directory
Feeds are saved in a file called rssFeeds.btf. This is a binary file, and it will go into your save directory
Feed Lists are saved in a file called rssFeedLists.btf. This is a binary file, and it will go into your save directory
There's a new setting - shouldTruncateAllFeedsToLimitOnSave - if you set this to true, then the save file will hold no more than the current max limit (50 by default) items for each feed. Due to a bug in all previous versions, the feeds were never properly truncated to that limit - which is why the save file kept getting bigger and startup time kept getting longer. Setting this to true dropped my feed file from 9 MB to 1.9 MB
Except for the settings file, all files are pointed to from the settings file. These pointers can be overridden by command line arguments, or by environment variable settings. These will all be documented in the Users Guide. For now, you can see the options here