Where does the Windows 8 Mail App store data?

Yes it does, if you go to C:\Users\<username>\AppData\Local\Packages\, open the folder that starts with "microsoft.windowscommunicationsapps", open the folders \LocalState\Indexed\LiveComm inside that, then select the folder with the email you want to look at, then one folder down you'll see two folders, "Mail" and "People". Select the one you want, and then a few more folders down you'll .eml files for each of it's stored emails/contacts.