the best performance add-on that I ever installed in my 1984 Jaguar XJ-6 was a K&N Air Filter. Now they have Oil Filters too and I sure plan to use them.
second to that was installing "Splitfire" spark plugs, though I know there are many negative opinions about those, but in my experience, they gave me instant starts, slightly better mileage, and more response. I later installed Bosch Platinum with the 4 tips, and they have been a total dissapointment, never again. I'm going back to Splitfire plugs.
Jose 
1984 Jaguar XJ-6
1965 Jaguar 3.8 'S'