There isn't going to be a significant performance improvement unless you are talking an inactive patient population of 25-50%.
As James points out, the headaches that then ensue aren't worth it unless you are willing to bet that you'll never need the data again.
With the new "improved" AC security, you can't have a second practice database running (for testing, archive data, practice, etc), so activating your archive DB will disable your production DB.
I doubt most practices want to do that during the day.
