In Reply To:
Why not take a test ride on a TitanFlex? You may be shocked how much longer you can hold your fancy new aero position when your bike absorbs all the road shock as opposed to your back.
I also have disc issues and never find any bike position to be affected by shock. But if I rode a comfort bike it probably would. Even on the hoods of my road bikes and my mtn bikes my torso is about 45 degrees from verticle so the verticle shock from bumps doesn't compress the spine.
To the OP: in my experience with me and friends, back troubles are so variable that you need to find out for yourself what position(s) you tolerate best. But FWIW, I am comfortable in a absolutely completely aggressive TT position. Completely horizontal back. This is because there's no weight supported by the spine...and I have extremely flexible hamstrings, so I keep a straight low back even with very tight hip angles...yet I only need a 90 degree hip angle on my tri bike because I'm using basically a 90 degree effective STA.