My mechanic measured the rear anti-roll bar on my 2002 XKR coupe and he measured it at 19mm. As a result, he thought getting the UltraRacing 20 mm rear anti-roll bar would be a waste of time and effort. However, I've wondered about metal fatigue on my now 12 year old anti-roll bar vs a new part.

In addition, I finally have a current status on the R Performance Pack parts made available from 2003-2006, thanks to Mark at at Gaudin Jaguar in NV (great part discounts via their website Direct JPLV Porsche Parts.com !).

The R Performance rear sway bar, part # Mna3500da, is no longer available, sigh.
However, a few front sway bars are still available (part #Mxd2100aa), and I have ordered one ($255 before shipping of 12.5% of the price). Be sure to get the matching bushing (part #Mxd2102aa), of which you will need two.

The springs from the R Performance pack are not available either. Apparently the parts began running out in the past year.
If someone can attest to their spring changes making a performance difference, I'd like to hear (the stock car's appearance never needed modification to my eyes, and I have the firm impression everyone who changed springs did purely for appearance).