I also purchased the MSA PREMIUM EXHAUST for Stock Manifold kit and have been experiencing loud popping at the exhaust on deceleration when lifting the gas pedal on 2nd and 3rd gear. I retarded the timing from 15 to 12 btdc and then to 10 btdc and the issue is still present. It came with an Xcelerator Turbo Muffler, downpipe tuve and 2.5 diameter aluminum pipes. I have a Schneider 274F camshaft and have Heard that people with the stock camshaft have no issues whatsoever.
This is really strange as i used to have the MSA TWICE PIPES kit and never experienced any deceleration popping.