"you must have either seen a completely different performance then I"
I think what you meant is "you must have either seen a completely different performance than I" THAN.
"Then" means "at that time" as in "Things we're better then" or "what happened next", as in "then the cat came in the door".
When you are comparing something you will use "than". "I had a better time than you."
I normally don't correct people and I'm only doing it because it seems to be an epidemic on the internet that people don't know the difference between THEN and THAN. I certainly am not a grammarian and my spelling is terrible but this bugs the hell out of me.
I'm only posting this because I will be away for a few weeks and won't have to read any responses or consider any hurt feelings.
Yes, we do need a third vampire musical.--Little Sally, Gypsy of the Year 2005.