I have made the mistake myself before of confusing stiffness and strength. A tube that is easier to bend does not necessarily have a lower stiffness, just a lower yield strength. When I bend 316 stainless tubes it is so much easier than 4130, but is equally structural as part of the frame (likely there is some work hardening from the bending that increases the strength a little in that area).
All the stainless frames I have built (all brazed with Fillet Pro) use 316 for the seatstays. Diameters are 8, 9.5 or 11mm depending on the application, with wall thickness from 0.5mm to 0.9mm. Test bikes for myself have used 9.5 x 0.5 and 8.0 x 0.7; both have performed just fine (I am 140lbs). I also use 316 for sleeves for ISP bikes and for custom stems.
Hope this helps!
thanks, Rob.