Maybe I'm just week, but even with just the roof top bars on, it's much
harder to raise the top.  My van is an 81, so maybe old age is affecting
it, (or is it old age affecting me) or my height (6'2") puts the bar at
the wrong height for lifting. I typically use my head in addtion to
pushing on the bars when raising the top (hey, that's using your head)
Anyway, I read about people lifting the top with layaks or roof top boxes
on top, but I know from my expereince, I could never lift the top with
anything else but the bars on it  - at least in the current postion on my
van.  Mine seem fairly aft though.  Rear bar aligns with the center of
the rear side window and the front bar is just aft of the center line of
the middle side window.
My $0.02, your milage may vary.
John