here's my ride with the factory roof racks on.
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Not sure about the 900 UK version. I have the US Thule Atlantis 1600. This is smaller about 39 lbs. Looks like the 900 is equivelent to the US Atlantis 2100 which weighs 56 lbs.
Here in the US, the Mazda CX-7 Factory rack in the catalog says that it supports 100 lbs. evenly distributed. So it seems like the 2100/900 would be ok, as long as you don't put more then 44 lbs in it.
Have you had an issues with the box on the standard rack. Thule doesn't seem to support the factory rack. They claim they haven't tested it and therefore won't be responsible for anything... whatever that means. The funny part is if you order the one from Mazda it is a Thule box.
The Thule box works fine on the Factory rack. I've had it on for 2 winters lugging ski/snowboard equipment. Like you said.. it is the same one that you can order directly from Mazda as an accessory. It is just cheaper to buy it from other places.