Re "folks are telling me it's normal to measure these differences", all I measured was -2db at 15 deg off axis, whereas you measured -5db. Some of this could be room effects, such as furniture, carpet, reflective walls, etc.
Also if you were off-axis vertically (i.e. VP150 points too high or too low), as you move laterally you'd get faster drop off. Just a possibility.
The off-axis problems of any stereo system (and the center channel is merely augmented stereo) are so well known Dr. Floyd Toole refers to it as an "antisocial" system -- it's optimized for a narrow sweet spot. IOW you can't have a large lateral audience who get equal sonic imaging, so it encourages solo listening.
Floyd Toole White Paper
To me issues like off-axis behavior are minor next to the huge differences of room effects, good vs poorly mixed material, sub calibration problems, etc.
