http://www.digidesign.com/digizine/a...ay04/techtalk/
That should answer a lot of questions.
All TDM plug-ins take 24 bits in, process higher, and output 24 bits. Whether or not the plug-in dithers is up to the developer. Not positive about RTAS.
Nothing is truncated (apart from plug-in outputs) when you use the dithered mixers (stereo or surround).