i agree, you won't get 5-10hp from a muffler swap..... Just like the dodge dealer saying the ram gets 28mpg........ Sure doing 70 downhill maybe. I have a flowmaster 50 series SUV muffler on my 4.7 ltr Grand Cherokee that my wife drives and it has a drone at 2000 rpm's and she hates it and wants me to put the factory muffler back on. We didn't notice and hp gains from that muffler swap. I like the sound of the muffler for sure, even at an idle it sounds deep and mean but i think that's the only benefit is the sound.
The dealer saying you'd get 28mpg is ridiculous, and no offense but you should have never believed that. All you have to do is look at the sticker for mandated, fairly accurate mpg expectations. I also never said a flowmaster would give you good hp increases. I said a straight through type muffler (like magnaflow) would. Flowmaster is hardly better than stock flow wise/performance wise.