Well, I have looked for Dyno charts for the F650CS, I finally found one hopefully a bit useful for the GS (same engine, but I don't know which software version was inherited by the single spark CS). It doesn't have multiple runs at different vacuum, but at least go low enough to be relevant for me (many charts begin at 3000+ rpm):
So it seems the engine still has torque down there, and it works indeed - but, should Teresa pull at 2-2.5k when given some gas, she still makes some pinging noise, so I generally don't go that low (except for a few mistakes). And in 5th she doesn't feel that lively under 3k. To tell the truth, this chart did not help me understand what should I call lugging.
One thing is apparent, torque begins to diminish over 5.5k - but 5.5k is merely there for emergency operation
