Am I correct in assuming that a deteriorating (sagging)front engine mount places strain on the exhaust downpipe and could cause a stress crack there? How much sag is still OK ? Also, what other mount ?

Hello, The straight answer to your question is "I don't know". I know there's a good bit of flexibility designed into an exhaust system's mounts and joints. My front mount lost half it's height during it's 112K service life with no apparent effect on the exhaust system. Since my exhaust system survived the unbelievably severe shaking caused by the IPD poly mount AND that posters who have reported 2 and 3 broken mounts never have had anything to say about their hot pipes, I'd conclude the allowable limits are broad. Kira