RE: How lite is too.....
5.5 gpp on a bow that's nearly 10 years old? I'd have to know several things: How well has the bow been maintained? How long has it been since the string and cables have been replaced? Does it already have any wear and tear problems that need to be fixed? How soon can you be prepared to buy a new bow?
Without knowing those things, and with the disclaimer that I don't care at all for the practice of shooting arrows less than 7 gpp on ANY bow, I'd say NO.