The 9X world cup is a very different ski from the 9X oversize. The world cup is a laminate construction with vertical sidewalls - no dualtec, pps lobes, no foam core... it is also quite a bit stiffer, and takes more muscle to turn it, and to get energy out of it. When referring to the stiffness, the regular ski may be longitudinally stiffer than the WC, but you also have to take torsional stiffness into account. On any world cup/race stock/race room ski the torsional stiffness is going to be much more than the shelf ski. The stiffer a ski is torsionally the more you can put into it on hard surfaces, but it also requires more effort to engage the edge. The WC will also have a more even flex pattern. All these differences amount to a ski that is designed purely for racing. Its shelf model counterpart usually takes the best qualities and tames them down so that they are easier to ski on. This leads to a ski that can make tight turns without a huge amount of effort, a ski that wont tire you out before lunch, and a ski that wont scare you to death on crowded slopes because it only goes one speed - fast (okay it might go two speeds - fast and faster - talking 25+ mph). The retail model ski is still capable of those mach speeds, but doesnt have to be skied like that all the time.
So to answer the question - yes the ski is stiff- its a race ski of course it is... is it too stiff to be handled? - no of course not. With the proper length and of course ability, any skier should have no trouble with it. You, as a racer BB, shoudlnt be too concerned with the stiffness of the oversize. It is quite softer than the WC 9S that you have been on, and it is a much smoother ride - especially in GS. As i recall from my 9X experience they are silky smooth, and quite fast - i skied a regular retail ski in a 181. Dont be afraid to go short either, a 160 or 167 should be plenty of ski for you. Alot of the girls i race with are on the 167 or 174 depending on their size. If you are an aggressive skier at 135lbs - go with the 167 - if you are still slightly timid in gates stick with the 160. Also if you are running slalom with them you may want the shorter length.