Moose,

Help me out.

The forces in the frame won't change due to the a sloping top tube. How they are absorbed may change. As to the reduced leverage, it would appear that you are referring to a relative twisting (torque) motion between the head-tube and bottom-bracket. No?

The reduction in leverage, in this case would cause the down-tube to provide more of the resistance to this torque because the top-tube was providing less resistence. With all else being equal - this might appear to make the frame more flexible, rather than less.

More likely, in my estimation is that the top-tube would absorb more bending and torsion, and the seat-tube would see less bending from this reduced leverage scenario.

How any of this translates to rear-triangle stiffness is very unclear to me. Especially, because there are many ways that the rear-triangle can flex, and its not clear which of them you're referring to.

In vertical compliance, making the seat and chain stays more nearly parallel will have the effect of softening the rear triangle.

If you're thinking of the rear triangle bending sideways, there may be some stiffening to the lowered stays, but it doesn't seem likely to be much - probably adjusting the diameter of the stays would have a larger impact. Also, if you look at www.sheldonbrown.com/rinard/fea.htm, you will see that the majority of strain-energy is absorbed by the down-tube, not rear triangle.

Torsion, or twisting of the rear triangle shouldn't be much of an issue. There is nothing holding the dropouts in place except the lateral resistence offered by the rear tire. This isn't a level of force that should cause concern.

Perhaps more important is the question of: Why is additional stiffness necessary or desireable?