My observations regarding the blower inlet ring and high altitude. I have roasted several hundred pounds with the SF 2lb model.
I'm at 6500' and was having too many beans burning or scorcing, >10%. After several discussions with tech support, they recommended removing the ring. It resulted in moderate improvement. I find that one side, the left looking at the chamber from the front, had great bean motion and the right side had none. The right side is where the scorcing seemed to be coming from. I closed the metal air vents under the chamber on the left side by about 50% but still could not get the right side moving at the start. The right side did 'fluidize' but not until well into the roast. My goal was to get it moving at the start like the left side.
The solution came by reducing the weight to 2lbs green weight or a bit less. Depends on the bean. Peaberrys need < 2lbs as well as most Africians. I put the factory supplied can away and use a scale now. I like a lighter roast, just before 2nd crack, I use preset 4 or 5 only with great results.
Initially I was PO'd that I wasn't getting the capacity the machine boasts, but it is what it is and I'll get over it!
Hope this helpfull to some.