plaster: Those seem like simple questions, but they're really not. It will depend highly on where and with whom you are racing.
Technically, your boat is required to be legal for Hobie class racing. Whether you will get protested or not is the question, because you won't have much chance of winning the protest. Finally, are you willing to take the chance that you'll have a good enough time that it's worth it even if you're disqualified in every race?
Note that, even sailing on Portsmouth handicap, your are technically required to follow class rules or take a rating hit for the non-legal equipment, weight, or whatever. You're probably not so likely to be protested compared to a class event, though.
You really need answers from the folks you'll be racing with. I'm sure there are areas where one doesn't even consider doing such a thing. Around here at non-Hobie events, you probably wouldn't get protested (until you started beating a lot of people!). Be aware that rules are rules, though, so even if everyone you talk to says it's no big deal, you can be protested!