One issue is aftermarket mufflers do away with the integrated catalytic converters so that reduces a lot of back pressure.
I would describe the 18" oval Del's that I have as a very "civilized performance muffling setup". I have the extra long baffles,,, which lower the tone a couple of more decibels. Usually,, I don't hear the engine at road speeds,,, only hear it on accelerations and engine braking (and it's not loud then, unless you are really on it hard). You get small but noticeable performance boosts from the weight reduction and improved torque curve (mid-range). I have found that it's worth the cost of decent used set (I paid $300.ca with no scratches). The Del's are not as edgey as the Two Bro's,, although I like those too. Have fun,,, cheers,, CAt'Delkevic - certainly louder than it was (what I wanted), but certainly not obnoxious.