Buzzski, What cheap adapters are you talking about? Something to mount the Aptus to the Sinar, or something to mount UW lenses on the Sinar?
In my experience, you won't be able to very wide with the P2 because of the limitations of the lens-to-back distance. Even with the bag bellows and recessed board, you wouldn't be able to do it. I use my Aptus 65 on a P2 with a Schneider APO 55mm Digitar in a Rollei shutter. It's wide, but not *that* wide.
The other issue for you is focus. You'll either need a sliding back adapter, or a Live View dongle.
I would be curious to experiment with different optics - enlarging lenses for instance, to see about some extreme macro shots.
You can connect the sync from a Copal shutter to the back just fine. LeafCapture has a setting for cable release.
Dave