Does it affect the performance of a portable generator to add a larger muffler to it?

I have a Baldor PC40H Generator (4000 Watts) It’s powered by a 8hp (GX240) Honda OHV motor. I would like to replace the original muffler with a larger quieter muffler!
Honda uses a flange style connection for their exhaust so I will need to come up with an adapter that will go to a pipe or tube connection depending on the type of muffler I get.

My brother works in a small machine shop & I’m sure he can come up with some type of a adapter for me! What I’m trying to figure out besides if it will affect the generator or not, What type of muffler should I use?

My understanding is: Some exhaust back-pressure is required, to prevent exhaust valve warping from cooling to fast. If the unit is to loud, consider all the noise-making parts. After all the rattles, whines and buzzes are addressed, the exhaust noise is all that's left. Air-cooled engines are louder, as they don't have a cooling jacket to keep internal sounds inside