That piece inside the muffler keeps it from collapsing when you tighten it, best to keep it there.
I believe he's referring to the little sleeve that's actually
inside the exhaust port behind the muffler, not the muffler's internal stabilizer or whatever it's called. These seem to be common on a lot of Poulans and some of the small cheaper Huskies that aren't much more than re-badged Poulans. I have no idea what they're for, unless they use the same cylinder blocks for different bore sizes and use the sleeves to reduce the port size on smaller diameter examples. Anyone else have any ideas? I'll go take a pic of one if people don't know what I'm talking about.
Personally, I'd leave it in unless you're planning on doing a port job. Otherwise you'll have very distinct ridges in the exhaust port which could induce significant turbulence in the exhaust flow, which is the last thing you'd want. That little sleeve doesn't actually go in all the way to the cylinder wall, so removing it won't really gain you anything as far as the port size goes - it'll still be just as restricted at the cylinder as before. Also you'd then need to enlarge the muffler's opening to match the now larger exhaust port.
I would ultimately like to do a port job on my 335 since it just feels like it would really respond to one, but I haven't gotten around to it yet. The mod to the muffler's internal stabilizer is a good one since it can greatly streamline the exhaust flow, but I also simply left the spark screen off altogether for a bit more of a gain. I think you'll really like how these things sound with a modded muffler. I can only imagine a ported one.