I had the upper oxygen sensor on the passenger side (code 1647) replaced today. Boy, what a difference it has made! It’s a different car!! Anyone experiencing sluggishness in their XJ8, check to see if the oxygen sensors are showing on your diagnostic reader. If so, get the offending sensor replaced. It will make all the difference.
Denso oxygen sensor from e-bay £55, fitted by independent Jaguar service centre £60. Well worth the money!