My take :
If the rules of the country say you must wear a USCG compliant pfd for you to sail offshore, whether racing or not, then you`re breaking the US law by not doing so. Separate issue from the Sailing Instructions altogether.
If the SI or NOR say the pfd must be USCG certified, it`s a closed subject. Don`t comply, don`t race.
My opinion on why the RO did not disqualify the girl in John`s original post : They would have to disqualify half the fleet from the regatta after the regatta was run. Not good for getting them to come back next event. They should have either issued a warning at the skipper`s briefing, or dsq-ed those guilty after the first race, and given them time to return to shore to change pfd`s before the 2nd race (not always possible, but a reasonable way to resolve this). By issuing a warning to only the winner, they have acted rather badly in my opinion, as the rest of the fleet did not receive the same treatment.
The question of which one is more comfortable, floats you better etc is not relevant, until you get the law changed.

Curious question : How does the CG feel about you floating at sea in a non-CG certified pfd when they pick you up, are you then liable for the cost of the rescue mssion ? Can you justify this by saying your CG certified one is strapped to the tramp of your recently sunk catamaran ? If so, you`d better be sure it sank.

I`m just contributing to this thread as I see two distinctly separate issues here : Obeying the rules of sailing, and obeying the law. When the SI`s refer to a specific law to be obeyed, then there is no room for not doing the right thing, even if it`s uncomfortable, which "safe" pfds almost always are, and life-vests even more so, unfortunately.