Collogues
I have spoken about this many times, to try and get best out of your beast is to unleash the beast, the best way is simply to replace the power hungry and woeful 3 speed automatic with a 5 speed manual transmission. I have also posted before here in Australia there is company in Sydney that do the conversion and it is a cheaper alternative to the ones that normally advertise and spoken off.
Many people will find just by putting a 5 speed manual transmission in their cars will be enough modification and give such a performance upgrade that searching for other performance upgrades may not be necessary.

link
Bellhousing Manual Ford T5 167mm Front Shaft TO Jaguar V12 Engine | eBay

I must also state that I have nothing to do business wise or personally with the seller of this product, I just simply see a cheaper way for a transmission transplant

 

Thank you just the info I won't to hear. I was thinking the t5 would be perfect but didn't know that there where bell housing available.
When and if I decide to go down the xjs route I will be putting a manual 5 speed in it.

 

The bell housing is $700, how much do you think the conversion would cost in total, what would you do for a flywheel etc. And where would you even get a flywheel?

 

I spoke to Jeff Dellow a while back before he advertised and from memory his kit was about the 2k mark Au$ that's without a box but a good box can be sourced for very little money, actually I had one lined up for $150, not sure if the kit had a Flywheel or not? If not they are readily available. The best thing to do is talk to them.
My Story
I was ready to buy the Dellows kit, but by chance I came across a
complete kit with another motor on line, "yet to be installed ( I wont to rebuild that 2nd motor first and then do a total transplant)" but most of this stuff came from dellows anyway, after seeing what's involed all you realy need is the bellhousing, gearbox, flywheel and pedal box. A second hand pedal box is about $300 and fits straight into the v12's from one of the six cylinder cars, there are also pedal kits to transform yours. Gearbox mounts can be easily fabb'd and a driveshaft altered for little money.
Talk to them, they make these bits, they know their stuff.