Just trying to see if there is any slight differences that might give us a few extra horses on the dyno.

I'm wondering if the older cams have more aggressive profiles than the newer ones (emissions grew more strict as time went on, is my logic behind this question)

 

I don't think the cams are the answer, but things can be done elsewhere.

PERFORMANCE FOR THE JAGUAR XK 4.2 ENGINE. / AJ6 Engineering

I suppose one could supercharge the engine. If your car is a US XJ6C, then swapping to SU carburettors could be beneficial.

Putting on a Series 3 head with its bigger inlet valves would also help.