Back on topic...... I think you would basically be throwing your money away John.
I read somewhere that the XF coupe already has more torsional rigidity than a 911 Porsche has (which is saying quite a bit).
Unless you plan on turning your Crossfire into strictly a "track" car, I wouldn't bother adding the extra weight, because it's really not necessary.
Save the money and put it towards another performance upgrade, or an appearance modification. I think you will get far more enjoyment out of something like that.