I thought you guys might get a kick out of this one. This race seems a little weird but it looks like the stock XKR-S jumped way early. I'm speculating but there may only be a 25hp difference between the two cars.

 

I'm not impressed with the differences. So small that it could simply come down to the exact shifting points in the two cars.

 

Obviously they didn't get the same start so that's not really a 1:1 comparison.

We don't know who did the tune, nor what the claimed output of the tuned car is.

But, having competed in a runway event and seeing what little difference 20-30HP makes in top end speed over a set distance, it doesn't seem too far out to me.

The tuned car definitely pulls away, and we are talking about 550HP vs. ~600HP? ~590HP? which in the end is a 10% difference or maybe even less...seems about right to me...

 

What we might be seeing there is the differences in a tuned car that moves the max power and torque to the high end of the RPM scale while actually maybe loosing some at the lower RPMs. That would account for both of the jump at the start by the stock car.

I know when I installed a free-flowing aftermarket exhaust on my '08 XK I gained (actually measured) substantial upper RPM (4000+ RPMs) power but, definitely lost power and torque at the low RPM ranges where most everyday driving occurs.