The XJ6 is megga heavy - if you could lose some weight in the car it would help make any tuning mods more noticable. Check the weight of the wheels and tyres! I bet you will be surprised! Big engines are gret for tuning they are very torquey. I tell people to drive it for a bit and decide where you want the power and then start tuning - you also appreciate the mods a lot more. I would go in this order (air filter, exhaust, cat, bigger injectors & fuel pump).
Obviously run some injector cleaner through and take out the gunk that builds up unless its new of course! I'm trialling some Power Boost Spray at the moment which is a foaming engine decoke - i'll post up some results when I get some Rolling Road time.
Lots of good advice from this ebook http://www.nettally.com/palmk/jaguar.html- its XJS but a lot applies to the XJ6! LEt us know how you get on. We have quite few JAG owners visiting our site - there doesn't seem to be much out there so we hope to become a popular meeting palce for Jag owners in this new forum.