Flip normals or delete geometry? For better overall performance This is certainly a noob question but I haven't found a solid answer yet. 

Say I have a 50 face, UV mapped object ready to be imported to a game engine. 

Only 40 of the 50 faces will be visible, the remaining 10 faces are still UV mapped

Obviously I don't want to render the parts that aren't visible so, 

Is it better for general performance to just delete the 10 faces  or leave them UV mapped with the normals flipped? If I flipped the normals, does that mean those parts wont render? Or will it make no difference?

Thanks.
As a simple, yet very true rule in game design:
- Remove anything the player will never see or use.

UV data is still loaded, vertices are still calculated, there is still a check to see if the face is visible. Everything except the final step, which is 'visualising the face'.