Every time I have tried tubeless offroad tyres I was always bugged with slow leaks due to clumps of mud, stones,tree bark etc getting between the bead and rim at low tyre pressures. I always use tubes now on Silverstones, Superswampers and Simex, but a couple of tube failures with sudden deflation at 30/40mph got really frightening, so now I glue the tube valve stems to the rim with silastic so that if the tyre is punctured it will deflate more slowly like a normal tubeless tyre.