Your torque figures are way off; 4.2 XKR is only at 416hp ,413lb of torque vs CTS-V 556hp, 551lb torque . Cts-v tested by magazines stock is around 11.9-12.3 at 116.9 mph yet he was trapping same as me and were neck to neck in this track. So once again, track conditions all very significantly. Stock 4.2 XKR would never stand a chance with this power difference.
I was told Bakersfield track is the fastest track in California and closer to sea level than others but turns out it's still 750 ft above , and with weather conditions that day, Density Altitude around 2000 it was only slightly better than other tracks. We don't have a see level track in California unfortunately.
I had 220 lb of weight when raced last time vs previous time had 50 lb less.
To figure hp there is no better tool than a Dyno, gives DA corrections automatically and if you know your starting stock level it's very easy to figure out power. My case: 359rwh with pulley only - 7rwh for pulley= 352 , percentage of base 416hp gives us 18% loss
Tuned figure 432rwh +18% gives us 509 with error +-7hp. Easy breezy 😉

 

Get the base 4.2 its more than enough power...

Hang on, you have been down this road before.
May I offer you a pedestrian's observation to the glaring.

You are consumed with fear of too much power and expensive maintenance.
This fear or challenge of such exotic ownership is the thrill you seek.
It would not be exotic otherwise.

That said, as several others have mentioned on this thread and tried to the first time around.
There is a raw-brutal power to the 510hp XKR that one never tires of. (perhaps because its wideband and visceral and effortless)
I have gotten bored with all others that were better on paper. They seemed like one-trick ponies that gave a burst of power on the bottom or the top and there is only so many times you can get impressed by the same song.
Perhaps because there is such dichotomy and contrast in the XKR between being a refined Rolls Royce like ride and the ruffian, that one is constantly impressed.

 

Sounds like a trip to Sydney might be in order!

I wish I could bring my car over. Your track looks so much better prepared than mine and we only have 6 meets a year so are at the mercy of the weather.