I have spoken to a fellow who raced on one (The owner of that boat has moved on to a J-145)The impression that I got is that they can be sailed to thier rating but it really takes a top notch crew on top of things all of the time.
J-boats seems to periodically come out with what I call their narrow series. I think of the J-27 as being sort of the first of that design philosophy. I see the J-125 as a logical extension of that series. There is real merit to a narrow boat but it comes at some big prices. In the case of the J-125 some of these compromises are less relevant bvecause it is designed to be an out and out raceboats sailed by a large crew. I am not a big fan of that kind of thinking because all raceboats become obsolete at some point, and you have to wonder about the value in creating a boat that is designed to such a narrow market. (Anyone want to buy a good used IOR one tonner?)