Yoshi end cans are decent enough quality. Not brilliant but not bad. The headers are too light are not good quality. American bikes run on very low grade fuel. U.K. Bikes use 95 or 97 fuel which is much harder on the headers. The weight of metal used isn't good enough.
Just so everyone knows. Stainless steel comes in two variants. One is ferros (iron) and will be slightly magnetic. The other, more commonly used is non ferros and so is also, like titanium, non magnetic.