Two-cycle engines normally require an expansion chamber exhaust system, tuned to the part of the power band where the engine is ported to produce the most horsepower (or torque, depending on what you're using the engine for) and they need more backpressure than a similar displacement four-cycle engine. Of course, this isn't practical for small engines on power equipment, but you'll see them on racing saws. If you reduce the back pressure too much on a 2-stroke engine, it will run worse, not better. Chainsaw engines are choked back, using a variety of methods including the muffler design, to meet EPA standards. If we want increased performance, we sacrifice some of that. When you use a chainsaw as much as we do, the low emission saw might end up throwing out more bad gas if it is forcing us to run the saw twice as long due to poor performance. We also can't always just use a bigger, stock saw to do the job (such as while in the tree)... so arborists and firewood cutters tend to go for performance and the EPA be damned. The EPA isn't the devil, the sale of these tools with small 2-stroke engines is through the roof, with most of the sales going to people who don't require this extra performance. They're just doing what they gotta do, and so are the OEM engineers.
If you do mods to the saw, it is best to go conservatively (as Jackjcc said) so that you don't end up having to buy a new muffler and starting over.
