Yukon and ClearH20Scott- my only deal with hopped-up saws is this: Ok, I can average 10mmbf/day currently in the small timber I'm falling. I'm running a Stihl 440 that I've put a lot of time and energy into with modifications. These modifications do allow the saw to run and cut faster, however I can only make facecuts so fast, I can can only run from the stump so fast, I still have to measure out every log with my Spencer tape before I lop it off, I still have to find my footing when I'm cutting on steep ground, I still have to stop and gas up the saw every 25 minutes or so ( used to be 45-50 minutes with my "stock" saw ).
Now, All the time and research I put into this modified saw I consider a good investment, because I learned a lot. However, it didn't buy me much as a pro timber faller. My body can only go so fast, My two hands can only do so much at one time. My chain still has to be razor sharp at all times, and as Gypo Logger mentioned, it's the chain that's doing the cutting.
Modified saws are a lot of fun, and don't get me wrong, I like running mine. However, I think there's a lot mental horsepower that goes into it as well.