A muffler mod is a safe way to enter the "modded" world. It's kind of like sticking your toe in the water to see what the temperature is.
It will give you imho a 10-15% increase in performance. After a muffler mod you just need to retune your saw. Mdavlee hit the nail on the head, the most important thing you can do to increase the performance of a saw is to really learn to sharpen a chain. There is a whole bunch of people out there who do not really know what a truly sharp chain is. I know I can file a chain to a point where it is sharper than factory out of the box. It took a lot of time and patience to get to that point, the key is not giving up. Find someone who can sharpen and have them give you some pointers if you feel like your not a good filer.
Like in anything else in life there are people who are very proficient in their trade and others that aren't, it's no different in the saw modding world. If your going to have it done do some serious research before having it done. There are work saws and there are cookie cutters. You don't want a leaned out high reving saw that sacrifices torque and bottom end.
Unless your cutting for a portion of your income, or just an enthusiast that has the extra cash to burn on a modded saw I really don't see much of a need to go past a muffler mod for increased performance. A good pro grade saw will cut more than fast enough for most people.
Thats another huge factor in how "fast" a saw gets the job done.
For people that run the 290's and 455's with the average chain, it shocks them to pick up our 361 or 044 with a really sharp chain and cut with it. The same with our 200T, most people cannot believe how fast the little saw cuts.