Velbon 730 - weight 2.35kg - load 8kg
Velbon 630 - weight 1.61kg - load 6kg
Velbon 530 - weight 1.41kg - load 4kg
Gitzo 2530 - weight 1.4kg - load 12kg
At first glance it appears that the Gitzo is incredible! Lighter, yet able to handle a heavier load than even the Velbon 730! And I know the 730 is superb with the 500mm lens. However (you knew that was coming, right?)... the Gitzo specs says this tripod is for use with a MAXIMUM lens of 300mm!
Gitzo is right that this tripod can RELIABLY STABILIZE only upto a FOCAL LENGTH of 300mm. Please note the portions I have put in caps, since it is not the weight of the lens you have placed on the tripod that matters, but the view-angle or the Focal length of the lens, that is of even more importance.
This is because of something known as "Torsional Rigidity" of a tripod. The load that can be placed on a tripod (12kg etc), should be separated from what the tripod can reliably stabilize, since these are 2 different things and only Gitzo seems to be publishing such dirty little secrets.
The Series-2 Gitzos (comparable to the Velbon 530, 540 etc) can take upto around 26lbs but are rated to reliably stabilize lenses with FLs upto around 200mm. 300mm is stretching it.
For 300mm etc., a Series-3 Gitzo (comparable to a Velbon 730 etc) would be a better bet with the exception that a Series-3 "systematic" Gitzo can stabilize upto a 500mm lens, reliably. This is because the "systematic" or modular series, have a top-plate with a larger diameter than other Series-3 Gitzos, thus enabling it to have a higher "Torsional Rigidity". Note that a Series-3 Gitzo can take a load upto around 40lbs, which has nothing to do with the Focal lengths of the lenses it can RELIABLY stabilize.
For lenses with Focal Lengths higher than 500mm, Gitzo recommends their Series-5.
We need to keep in mind that only a pro-oriented company like Gitzo publishes such statistics about the load capacity of the tripod versus the Focal lengths it can stabilize. Several companies just publish the load capacity of the tripod and its weight, with none of the underlying fundamentals behind taking a stable (critically sharp) image.