I have a 512GB thin back for when I get my 512GB SDXC card.
Sweet! eBay? And if so, do you have the link to it? I'm curious.
The Rockbox tool is pretty straight forward, just plug in your device and it will detect everything for you. Plenty of YouTube on how to do this simple process. Also, as I said, I have had some trouble with the 2000mah battery upgrade. If you still wanna try this, I would at least advise ordering a regular battery in addition to the 2000mah battery in case the 2000mah battery doesn't fit like mine didn't.
Regarding Rockbox, I'm not worried about the procedure. What I'm curious about is whether there is any functional difference between how it runs on an 80GB thin classic versus a 120GB thin classic or a 160GB thin classic.
If there is no difference, then I'd imagine (especially since the 128GB format limit doesn't apply to Rockbox), there's no reason to not just go with 80GB classics all the time for that as it'd probably be way cheaper than going the 120GB thin or 160GB thin route.
As for the 2000mAh battery, I'm asking you if you were using it with an mSATA configuration or an SDXC configuration. It makes sense that the latter would not fit that battery, however, everything I know of the former configuration and what I've read elsewhere suggests it ought to work, hence me asking which configuration you were trying to use with it.