I use my iPhone to measure the camera tilt angle all the time for high-end VFX on big budget jobs. When properly calibrated it is just as accurate as my $650 digital inclinometer.
A built-in accelerometer would be even more accurate by removing the variable of "was the measuring device positioned in exactly the same way for each measurement."
As far as WAAS-augmented GPS, surveyors use it to align bridges and buildings within millimeters. If your FX guys can't adapt to small errors like that then you are using the wrong FX guys.


