Just did the 1.6 upgrade and so far so good. Memory usage is up to 54MB in 1.6 from 50MB in 1.5.2 – granted I don't have as many notes as you, and I don't store PDF's within Evernote, so hopefully I won't suffer from the same issues as you have. I'll leave it running all day (as I usually do) and see if the memory spirals up. I have the disk image for 1.5.2 handy if you need it.