Trimbowly

Hi guys new to the forum I just bought a BA T auto ute I noticed she had a bit of a grumble when ideling and crusing under 60-70km and poked my head under to find out the center muffler had been romoved and replaced with press bend pipes I am wondering do I have more power or less than standard as there seems to be alot of conflicting stories ?? Also what other mods can be done to make more power with out the loss of fuel efficiency ?? regards Po