Yes, did a raim prediction before using on a GPS arrival. Computed that raim wasn't available. Ended up being vmc below LSA anyway so the steps weren't required, however the GPS continued to perform normally the entire time.
Doubt you can fly low enough to have terrain shielding when it comes to GPS. Never heard of that reducing GPS navigation accuracy. Have a look at the Queenstown approach charts. You would have to be flying in a tunnel for that to be an issue.