I am up in the air it sounds great http://www.youtube.com/watch?v=kEzf78tq5oI but I can't get over reading this.

Resonator does produce back pressure with the crest and bumps, sometimes its perforated. The crest/bumps or perforated holes inside the resonator act as speed bump on the road which cancel out /slow down the sound waves. Hence, quiet down the exhaust note.

Removing the resonator will have negative effect on the exhaust valves, since the exhaust valves need back pressure to assist it's open and close motion. However, if you retain the rear most muffler, most of the back pressure is still there. Your mechanic is right in general. But most people do not care about the minimal ill effect on cars to achieve the personal preference with cars. if u do the research u will find that this is true, how ever just removing the resonator may not have a strong enough effect on the back pressure to make a difference, but non the less it will make a difference.