Termis on the 996 and DP pipes on the ST4S.
I must admit I did think about getting some road legal carbon cans for the ST, and it does concern me how loud it is.
As others have said though, going through villages on low revs is the best bet. Trouble is they have generally heard you coming well before you get there
If I could get some inserts for the cans I would consider buying some for the times I want to use stealth mode. Makes much more sense than swapping the cans over anyway.