Oh yeah!
There's a whole pile of folks that make go-fast stuff for the V-12....
AJ6 Engineering, where Roger Bywater, former Jag engineer makes his home, Gran Turismo Jaguar, Keisler Automotive, Terry's Jaguar parts, Rob Beere Racing, Forward Engineering.....this is just off the top of my head and there are others.
BUT.....you should get the car fully sorted back to original specs and see what you think of your car as it was intended before you go the route of aftermarket hopping up.
IMHO the carbed XJ sedans are brutally powerful and fast when they are set up as they should be.
Jeb