You will not be disappointed anyplace with the Echo CS-510. They are pound for pound the best of the modern Echo saws. No need to mod one anyplace either, but they do respond well to opening up the muffler a tad.
As with any other peice of power equipment from any manufacturer, tune it EXACTLY for the fuel you use, weather, altitude, etc.
We just finished a big commercial job a few days ago, taking out about 50 trees total, from about 10" in diameter to just over 24".
The three saws that saw the most use on this job were the Echo CS-360T, Echo CS-510, and my closed port Husqvarna 55.
We used the Husqvarna 181 to take down the biggest trees, and cut up trunks. The smaller saws did all the limbing.
I attached a pic of my helper Nathan with his favorite saw from my line-up, the little Echo CS-360T. It flat ROCKS, and we've been running the crap out of it for quite some time now, and still no "death rattle"!....LOL....Cliff