I have recently bought a low mileage (40k kms) saloon and after an XKR feel the difference in mid range torque.
I am curious about the Chip Tuning offered by Quantum (A$1300 through local AUS distributor). I gather they take readings and send to UK who then send an upgrade they install electronically.
They say 8% increase in torque and power.
Love to hear from anyone who has experience of these.