Rear spring stiffness?

Personally I have no clue. RaceTech's database doesn't show the 821 either. You could always try giving them a call to see if they know.

The manual says this under rear suspension specs:
SACHS:
Progressive.
The shock absorber is adjustable for rebound and spring preload. At the bottom pivot point it is connected to a die-cast aluminium single-sided swingarm.

Stroke
61.5 mm

Stroke (Low Seat)
55.5 mm

Spring preload
0 mm from Min. (all unpreloaded)

Wheel travel
150 mm

Wheel travel (Low Seat)
133 mm

i read somewhere it was progressive 75 to 140. or there abouts. The SP has a 110n/mm stock.