Most seem to state ‘fashion accessory only’ or ‘just for psoing with your surf board’ with either no or low (~40kg) load rating. The highest rated rails I have found are these ones on ebay at 75kg.
http://www.ebay.co.uk/itm/200904413155
And you can get low profile cross bars for them that don’t raise the height much above the rails.
You can get more heavy duty cross bars that fix direct to the roof. A pair of cross bars attached to just 4 of the fixings (2 each side) can be rated at 100kg, which is more than any rails I have seen which attach to 6 fixings (3 on each rail). So it seems like the fixing points are the limiting factor, although 4 M6’s doesn’t seem a lot.
I’m leaning towards the ebay rails above, as I don’t really want cross bars ‘permanently’ attached. Will just attach the cross bars to the rails when I need then. I’ll probably also go for some thule type cross bars that clamp on above the rails to give me a bit more clearance for the curve of an upside down canoe.