I run c-clip eliminators on my car with no leaking issues....on an 8.8. My kit came with a gasket to help seal things up....not sure if they all do or not.
I looked at going to a 9" but decided to beef up the 8.8 because I was only going to make about 500hp. I was concerned that with only 500 hp the 9" may slow me down because of the added weight or power needed to turn it when compared to an 8.8.
...can't go wrong with a 9" though.
10.91 @ 124mph (60'....1.44)
Stock Short Block, Stock T-5, Stock Computer, 93 Octane, A-Trim....driven like it was stolen