It sounds like the local track, or, more likely, the people setting the testing standards, are not doing things correctly, when it comes to the sound testing, between the 4ts and 2ts. Unless you're running a short muffler / badly packed / burnt out muffler, a later model (and not so later model) 2t should be able to get lower sound readings than the majority of std piped modern 4t MXers.
But, that's besides the point. The noise standards / testing, is what has been set by whoever it is that set them.
So, meet them. Unless the standards 'they' require, are outrageous. Give 'em an inch, and they'll take a mile? Yes, it can happen. But, "give them an inch, then use the results to stop them from taking a mile", is a pretty good tactic. Meet a reasonable standard, then fight the ****ers from a stronger stand point. It can, and does, work, with dealing with a lot of mongrels that want to control things. It's worked over the years, and very recently, for my friends and I, with regards to dealing with councils and environmental 'police'.
A quieter muffler required - fit one.
A lot of extra noise beyond what is coming from the end of the muffler, comes from resonance within / of the expansion chamber. Pipe guards - fitted properly, can attenuate a lot of noise.
An old tube, over the chamber, can seriously reduce chamber noise. Daggy?, shit yes but cheap, and with a bit of experimentation, not that bad in producing other pollutants, such as burning rubber.
Things that vibrate - there's a Lot of them on a bike, especially a non counter balanced 2t - such as exhaust springs / loose pipes, even the shock spring . Just revving your bike, and looking at the shock spring can have you seeing big vibration / hearing a lot of noise from said spring. Just rotating it can help, before changing pre-load (think of the spring as a tuning fork, and you'll have a basic understanding of it, and it's 'frequency changes - of course, set it back to what it Should be, if you need to take the extra, or less, pre-load route, when you actually ride the bike). Foam in the air box vents, can lower noise, without messing with your bikes power - intake noise, is a big component of noise output.
As for the "idea" of the extended stinger into the convergent cone of the chamber - well howdy - it's been done for years, by those of us that know of it, and, funnily, it's recommended in a couple of "old school" 2t tuning books that come in for a fair bit of ridicule, from a few here, quite often. I think you'll find it in Gorden Jennings book, in the section on exhausts. It works well - I've done exactly that to most of my 2ts I've had over a fair few decades now. Even if they have been "track only" bikes, be they dirt, or tar. I much prefer to have a quieter bike, over a louder one. Anyone with a bit of grey matter between their ears, usually does.