I have Keihan stainless steel systems on both my R80GS and R8G/S, the latter of which is the same one as your ST, they look exactly the same as the original, have a slightly deeper exhaust note and come with a 10 year warranty but I expect them to last a lot longer and some parts already have. Stainless does go yellow with time but can be polished back to original very easily, I recommend this option rather than coating your orginal system which will still rust out eventually.
If it's in the Touratech catalogue then you don't need it
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