Sorry to dig up an old Post, but it has a lot of good info. I just acquired a sp81 and have 75% of the parts to build a sp80. To answer the original question the sp80 doesn't have a chain brake and a slightly smaller clutch cover. This is probably the reason for a slight weight advantage. I was shocked at how light the saw is though for its age.
I also noticed that at least my sp80 has a hi and low adjustable carb while my sp81 only has a low speed screw.