dielectric diagonals

I see that William Optics has a "Durabright" dielectric diagonal
with a 15mm thick mirror, instead of the 10mm thick mirror of
their standard dielectric diagonal. Is the thicker mirror
supposed to prevent warping of the surface due to the many layers
of dielectric? Is warping a known problem, and if so, how bad is it?
Anyone here have any personal experience with this, or can anyone
point to any studies or anecdotes?

About half of the dielectric diagonals I have
tested (regardless of high-end maker) are
warped to some degree. Its generally a minor
effect, and probably not noticable in most
situations. On the other hand, if you got a
0.99 Strehl super-duper APO and do some careful
star tests with your 3mm eyepiece, you will see
the astigmatism from the diagonal, and not like
it very much.

The effect will be worse in fast scopes like
F/6 or F/7, since the light cone is bigger at the
mirror surface. At F/10 or F/15 (or F/20) it
will be much harder to see any problems.

I usually just write some notes about the wavefront
errors on the diagonal, and try to use the
good ones in the fast/good scopes. And the
rest are fine in the SCTs at F/10, etc.