"I understand the Hybrid SACD have a SACD layer and a red book layer. Assuming both layers are from the same mastering could a CDP like the Isis reading the red book out perform or be as good as say a Modified Modwright reading the SACD level?"
I think the biggest mistake made with SACD, was to allow both single and dual layer discs. If Sony only allowed for hybrid discs, people would have been building their SACD collections even if they didn't know what SACD's were. The format may not have failed.
As far as quality goes, that's something you have to decide for yourself. I thought CD's sounded better on my Wadia than an SACD of the same recording, did on my Sony 9000ES. But its a judgement call that you have to make.
"Do people buy SACDs for the red book layer? in theory it should contain the same mastering."
Yes, but unfortunately most SACD's are single layer.