I'm just under the impression that the CAAD 9 is going to be too aggressive for me no matter what. I have a feeling I'd be much better served by something like a synapse.
I really do like that Trek 760 I have and I think it will definitely take 28's (I think it has some already). Perhaps a good first step might be to put that bike back into a geared setup and sell the CAAD 9 frame to get started? It's all steel though. No carbon fork etc. Pretty damn flexy 531P tubing.