Question:
How much HP does an exhaust add?
Answer:
None
Ideally, and exhaust system is setup to efficiently scavenge exhaust gasses from the motor. If it's too large, the efficiency is poor and you loose HP. If it's too small then you're restricting the exit of spent gasses and you loose HP.
Cat-backs in most cases, do little or nothing to remove restriction or promote efficiency. They change the exhaust note. That said, there are OEM cat-backs that are restrictive and are undersized for the purpose of meeting an emissions requirement.
If you REALLY want to improve exhaust efficiency, then get and install a set of long tube headers. Matched with the right cat-back, the efficiency will be improved and it has the added benefit of deepening the exhaust note.