This is for your wife, right? I'm going to go out on a limb here and assume she could not be competitive as a sumo wrestler, based on the fact that she's a female roadie. If my assumption is correct, she's doesn't need a crazy stiff frame. More stiff doesn't mean more fast, about 99% of the power you make goes to the rear wheel on any modern bike, if stiffer frames really were faster they'd be marketed with their watt savings. Stiff frames handle predictably, and stiff is relative to the stress the frame will see, tightly correlated to rider weight.

So take the weight savings and the cost, and decide whether it's worth it to you.