Wrong Walker muffler

Does anyone know if there's any difference between a Walker heavy duty muffler for $70 and a walker mega flow muffler for much more $$$ other than the sound? I ordered the wrong muffler and got the heavy duty muffler instead of the mega flow but the heavy duty muffler is a full flow muffler also as far as I can tell since I can look in one end and out the other. I have not noticed an increase in mpg yet but I have noticed a better throttle response. I called walker tech support and they said I need to buy the mega flow to get better mpg but they may just want to sell me another muffler. If the sound is the only difference I will stick with what I have and save some money.