I have ran a C4 for about 700 miles now. I rebuilt it myself with the B&M kit and used the B&M deep pan. At this time I do not have any leaks. I had a leak at the dip stick, I had to modify the DS hole on the pan for a Lokar DS. I ended up using JB weld and it worked. Now if you are leaking from the pan gasket, if it were me I would us good gasket maker and the gasket to seal it up. If that doesn't work you may need to mill the bottom of your transmission to level the surface the pan seals against. I don't know any reason why that surface would warp.
The only real truth I know is that my transmission is not leaking.
If I'm way off and there is a much better way I sure would like to know it.