First off - if you think Dpad is more accurate for steering, go for it! Personally, I just wouldn't want to adapt to harsh digital input, as opposed to smooth analog control - even if that means I won't be as perfect.

Answering some of your questions:

Yes, L1/R1 are fully pressure sensitive. Even finer than L2/R2, imho, because they are not as springy.

The Dpad is, as the name suggests, digital - not sensitive.

The rumor that the DS3 vibrates when pitch is wrong pretty much proved to be nonsense, sadly...

I believe you can turn off the DS3 rumble in the XMB, if the game itself doesn't allow it.

Concerning your Sideshift question: I can't really say if I would use it more on lower speeds, since I don't race lower speeds ever since I finished campaign. I do, however, use sideshift very often, but mostly to get the last bit of a turn, rather than re-adjusting my raceline after a turn...