Lol, Fenril, I don't hate you.

You are correct, a background FPS of 20 will halve the amount of data sent. Not good really... Jamba uses this library: http://www.wowwiki.com/ChatThrottleLib

Easy fix is to set your background FPS on everything to a minimum of 21.

Medium fix is for you to edit every occurance of ChatThrottleLib.lua in your wow addons folders and change this line to a lower value:

ChatThrottleLib.MIN_FPS = 20 -- Reduce output CPS to half (and don't burst) if FPS drops below this value

Harder fix is for me to work out how to override the library in Jamba.