I've never needed to overexpose E6 for processing but then again, I have done much less of that and more going the other way (Color Neg in E6 chemistry). In the case of processing color neg as a transparency, you will definitely need to over expose by 2 stops. In either event, there is an easy way to find out. Just find a lab that will do "snip tests". Just shoot your film at it's normal EI, have the lab run the snip, evaluate the snip frames, then push the rest of the roll if need be.

-Mark