I dont know what the laws are over there, but if you go for a 100cell cat, and you get caught, you are up for a $20k fine, maybe more. And its not hard for a cop to get the EPA down to test your exhaust emissions on the spot, thats how they find you out.
hi flow is exactly that, the less the cells, the higher the flow (but also the higher the emissions), sports is pretty much the same definition.
Never heard of boost creep so cant help you there. My opinion... Just play it safe and get a 200cell which is perfectly legal and you wont have the worry of "will i get caught or wont i" as its more than enough, and i guarantee you will feel the difference.