Most Ion damage I've seen has been confined to an area in the center of the screen, usually about the same diameter as the inside of the neck, the 12JP4 is probably the most notorious for this condition.
An example can be seen in this thread from Isthmus' Andrea restoration thread here:
http://i1216.photobucket.com/albums/...pssy8t0jtj.jpg
I think the Ions only travel in more or less a straight line and can't be deflected to the outer areas of the screen, could be wrong about that but I don't think it would cause a completely dim tube.