Thanks to pureIso, who provided me with an EA MMA checksum fixing tool, I can show you how to max out your created fighter.
MMA Fixer provided by pureiso
1. In case you don't already have one, you need to create a fighter via the "fighter share" menu. From there you could also import your career mode fighter.
2. Copy your "MMA_FIGHTERS" fighters file on your PC and open it with an hex editor.
3. Do a text string search for your fighter first or last name. http://imageshack.us/photo/my-images/7/picture1uzv.jpg/
4. Change your values according to the picture, but make sure not to overwrite the quote signs. http://imageshack.us/photo/my-images/97/picture2wi.jpg/
5. Save the file, close the hex editor, drag the file into the MMA fixer. Press ok & copy the file back on your Xbox.
Now your fighter's attributes should look like this:
Unfortunately, I haven't figured out yet how to do it with an career mode fighter.