I'd be concerned about lateral forces, sudden stops, and such.
The cross bars (the bars running perpendicular to the rails)are generally the weak link....the cross bars are only rated at 130#'s..:Wow1:....but I think the actual rails themselves (in my case a 5th gen TE 4runner) will hold much more. If you look at the Rocky Road roof rack it looks very substantial. I do think the roof bars themselves are capable of different ratings. Check all connections first, the strength can only be as strong as the bolts holding it down.