500

The stock ones are stainless steel too. I recently removed my originals and cleaned them up. No rust at all. S/S exhaust parts can "brown" but no pitting corrosion (the SSIs will do this too).

SSIs are definitely nice, but the stock heat exchangers are a quality part.

Having said that, they are old and it is possible to have a crack that can, as you said, result in exhaust in the cabin. But I don't think that is necessarily common at all.