Thats quite a jump in machines also. I cut a field last night with a neighbor in his new 9770 with a 35ft Macdon draper and in 65 bu wheat we were running at 4.2mph to 4.9. I generally only look at rotor speed though.
We had a 36' Macdon (actually 35') draper head on both 9770's and 9660's. Saw no practical difference in how fast the 9660 takes the wheat and the 9770 where the yield is modest - 25 to 40 bushels.
I've been told that the STS technology actually works better when there is more wheat in the cylinder unlike the straw walkers and chance of throwing grain out behind the STS is less when pushed. Also have heard that 5mph would not be unacceptable in 20-40 bushel wheat, and know folks who run between 4.5 and 5mph under those conditions.
Interesting on low speed in 20-40 bpa, I would have thought you could run 5+ mph, in the 6 + range, if you are just cutting for the heads and not running through all/heavy straw. And, maybe that is what you mean. On our 9x10 walker it likes it fast in the lighter yields, and this year's conditions could run 4-4.5 in 50-60 bpa.
9650 with 36 foot honeybee cutting 45-50 bushel durum wheat that is 30" high and leaving 12"stubble behind at 5-5.5 mph. 9770 in same conditions with same head is 6-6.5 mph.. Durum wheat will thrash easier than spring wheat and this is mid day thrashing conditions ... These are not assumtions this is how it is on our farm...