I have the 55 Rancher - 53cc job. Non modded. Bought it new back in high school about 8 years ago. I've cut tons of wood with it, and it won't stop. It's my go-to saw and always starts no matter how cold.
I know it is not a pro-saw, but I run it like one. Biggest tree I've cut with it was a 30'' Black Oak, and it did fine. I've flushed many stumps with it, and noodled plenty of big rounds. Just keeps going. I keep the filter clean, the chain sharp, and use quality oil. Definitely not as fast as a pro-saw, but a great all around machine.
I still run the 3/8 pitch chain, and even though it has the 20'' bar on it, she still does good. I know the narrow kerf stuff is better, but if you keep the 3/8 good and sharp, it does pretty good. Someday I will buy the .325 for it, but too lazy right now.
Peter
Stihl 056 Magnum II (milling saw)
Husqvarna 55 Rancher (firewood/everything)
Shindaiwa 360 (trimming, limbing, etc)