Tyga pipes have the manifold stubs which bolt to the barrel using the oem gasket, I put sealant on this joint for good measure. Then the pipes slip over the manifold (as per Andy's link), they definetely dont bolt straight onto the barrel.
I did consider turning grooves for o-rings in the manifold stub but I thought the wall thickness of the stub was a bit thin for this on the Tygas.
I have had no major issues with oil leakage using sealant, nothing worse than a bellypan full of sticky oil goop! The JHA's that were on my bike when I bought it had some major leakage issues