There is no "hole in the head" after drilling. Surgeon drill the outside layer of the skull for embedding the implant. So that's not making hole. Surgeon does drill into mastoid bone to make cochlea accessible for insertion of electrode array into cochlea.
after the surgeon is done, you don't see hole. 
AudioFuzzy, please find my post in other threads regarding the residual hearing loss after implantation. (hint: cochlea is fluid filled) click on my name and select "find more posts by" me.
Of course, deaf person is deaf to begin with regardless of residual hearing. That's why there is criteria to qualify for CI is that if you CANT benefit from Hearing aid then the residual hearing is not there. If you CAN benefit from hearing aid then chance are that there is plenty of residual hearing left then the client will be suggested to go the hearing aid route till his/her hearing loss worsen and lose all of residual hearing over the year.
In other word residual hearing are NOT greatest tools for those struggling to hear with hearing aid because not all of cillia in cochlea are NOT there as standard in the process of child development. For example, I have tiny residual hearing in the low frequencies in range of 250 to 400hz at 70db that's it. and have none in rest of frequencies that goes up to high frequency. It is as if the area in my cochlea where cillia are like a field of grass mowed down leaving both side a little left over cillia.
