on a true dual exhaust, you will lose power. The engine simply does not provide enough exhaust backpressure or velocity to warrant needing dual exhaust.
On naturally aspirated engines, backpressure is a necessary item. Backpressure and piping diameter create exhaust velocity. By making an exhaust too large, you will lose your back pressure and exhaust velocity. That will result in lost power.
Forced induction engines are a different beast though. Because of the excess amount of exhaust gases, the gases must be expelled from the piping as quick as possible, turbo'ed engines especially. Truly, the best exhaust for a turbo'ed engine is none at all.