Battery size is more of an aesthetic thing
it rarely matters since the battery lasts so long anyway. If the battery indicator is half-full then you still have a good 14 hours of reading time.
Fonts are something that does matter to many people though.
I wouldn't suggest using the serial port if it didn't have one
I suggest you take a look at our Kindle Developer's Corner.
The jailbreak merely installs the MobileRead signing certificate for additional software updates hosted here, it does not replace the OS.
The serial port is hidden underneath the casing.