Jim,
May well have discussed several times, but not easy to find every post across all the sub-forums !
Anyway, I looked at the SLK320 forum and lots of stuff there about removing secondary cats. Common theme is they were fitted by M-B to pass California emissions and highly recommended to remove if not needed to pass a test. Also, agree with you, no real performance gains. What's interesting, 2 ECU re-mappers in UK (inc Eurocharged) recommended their removal before the re-map ?
Luckily I have a specialist exhaust shop (LongLife) 1/2 mile away from me and they do re-mapping as well as cat removal service. Am taking the car to them to see what they can do. Basically I want the re-map for the extra power/drivability etc. and a little more noise, but no too much !
Mick