Performance up-grades XJ6 series 3??????

Plodding along in my '82 XJ6 since 1986 I can't but help wondering if there isn't some more power available from my venerable 4.2.
Despite the potential thrills, I really don't want to drop a Chevy 350 into my engine bay.
My thoughts are that the intake is restricted by a nozzle 1/3 the size of the air port. And, the exhaust is also restricted by early catalytic converter technology.
Has anybody experimented with either a better airflow in or exhaust flow out of an XJ6? Are there other options? I ask because the tech pages in the magazines ignore me and my mechanic laughs at me.

I heard/read somewhere there is a supercharger made for the Jaguar six, but it may only be for c, d and e-types. Have you started by exchanging your air filter for a K&N? That will give you a HP boost with no downside other than cost and fuel consumption. I bet you could also have the ECU reprogrammed, install a better ignition system and replace your injectors for a high flow type.

I have a K & N air filter in my '84 XJ-6. There is some increase in horsepower and fuel economy but nothing to write a book about. The 4.2 engine is not a red light V8, it's an engine for high speed cruising, where at certain rpm's, there is a "sweet spot" where it becomes "silent".

I also added Splitfire spark plugs, p/n SF6E, I get fast starts, and when I step on it, the engine responds with more fire, though there are skeptics.

Otherwise my engine is stock, except for some mods I made based on Jaguar technician recommendations. Some people remove the Air Pump and related parts, claiming a bit more power, but I stopped trying to out-smart Jaguar engineers a long time ago.

Ex

A lot of mods to the series 3 come from the XJ 40 engine. I have seen the manifolds and injectors used along with the engine management systems. AJ6 Engineering in UK has a lot of upgrades for the series.

I do have K&N filter. I really don't see much in the way of an increase in performance.
I realize that the XJ6 is seen as a great cruiser, not a speed manic. However, I think there is more hiding in there. Seems like the power band is from 2500-4000 rpm. I don't need to squeal my tires, ut I am tired of being dusted by 1970 Toyotas.
In any case thanks for any suggestions!