The muffler mod thing is for kids. It will wake up a saw, but at a pretty high price.
For one, you will lose gas mileage. If you're like me, you hate filling up the saw. Once modded, you'll be filling up often. If you used to cut up two big logs on one tank, now you'll only do 1.5.
If you're a climber, your topping saw will run out before you are done with the tree. Is it faster if you have to lower the saw to have it refilled?
Second, where does the extra gas go? Into the environment. Unburned gas is a VOC, pollution. That's how it makes more power, by moving the charge forward, thereby spilling fuel into the exhaust.
Third, is the noise. If you study on hearing protection, you'll learn that ear plugs and muffs only reduce decibels. A stock saw and ear plugs get the noise down to a safe level. Once you open the exhaust, now you need to wear both good plugs, and good ear muffs, at the same time, to reduce the decibel output to a safe level. Who is going to do that? Very few know they have to, even fewer will do it. This means muffler mods cause hearing damage.
One of the guys said about the modded 066 the other day, "I used to be able to cut up a whole tree, now I gotta stop and fuel the saw to finish, is it any faster?"
If you want to have a fast saw, learn about chains, and cutters. There is much more to be gained there than the muffler.