I have put roof rails on my Austop. From memory the recommended max loading suggested by Austop is fairly low though. We were thinking of a tandem sit on kayak on there at nearly 40 kg and I think that would have been pushing it. Got the rails but never did try to struggle with getting the kayak on there! Occasional use now likely to be inflatable paddle boards, 10 to 15kg each.
When fitting I used the method found on this forum somewhere. Attach the fixing bolts to the roof first with suitable sealant, with the threaded end pointing up (bolt head under the roof), then fasten the rails to the bolts.