The reason I didn't include an internal TOC or hyperlinks was because on the PRS-505 it takes almost as long, or longer to select the correct link and activate it as it takes to just jump back to the external TOC.
I agree that an internal TOC would become far more useful with the touchscreen ebook readers.
Good luck creating your version
EDIT: Hmm, just looking at your TOC idea, would you mind if I end up borrowing it (or something similar) for mine?
Also, I don't know how the PRS-600 manages large ePubs compared to the 505, but speed is an important factor. It should be split into as many xhtml streams as possible - I ended up having a separate xhtml file for every chapter in the bible. And be careful when you build your toc.ncx file - you should reference the file itself and no more. Don't try to link to a specific spot in an xhtml file unless it's absolutely necessary (eg: verses).