Corona is infectious and (rarely) can mutate into FIP. Some cats remain carriers for life, but others, like the Siamese you mentioned, will not. I think on Dr Addie's site about FIP, a cat who has consistently high titres at each test over the course of a year can be considered a carrier - heres the site
http://dr-addie.com/ . If the titres go down to zero or below a certain level (can't remember exactly) then there is no risk to other cats of catching corona. Corona itself will usually just cause mild sneezing and sometimes persistent diarrhea, although rarely neurological symptoms, like with Gwynnie when she had back leg paralysis - although we are still not sure that it was down to corona.
I didnt retest Gwynnie after she was really poorly and had high titres though, but she's been completely well for 2, nearly 3, years now.