It sounds to me like your car preferences are quite close to mine, and I can tell you (having recently owned one) the S14a will drive you nuts, as soon as you begin modifying it, the tiny amount of GT it had disappears.
That's how I ended up in an X308 Jag XJR, I went for it because my budget bought poor X350s and I ended up getting a really mint car for a tiny amount of money. It does mean that I can shed some light for you.
The autobox is an interesting one, its a Merc unit in the XJR to handle the extra torque. I wouldn't call it a stupid gearbox, but it certainly isn't DSG.
For day to day rolling around, it's superb, very smooth and you hardly know its even there, and given a sharp kick to the floor it will launch the car at full attack without to much hesitation.
For sporty driving it's an interesting one, normally it moves off in second gear, so you only get first in sports - which makes for a breathtaking launch, the manual selection is sufficient and holding 2 makes an awesome noise. It can sometimes be a little daft coming out of corners at full attack, but never changes up and bogs down (in my experience).
As for being a barge, making sure you get one with CATS adaptive suspension is a must (standard I believe on a UK XJR, just make sure some cheapo hasn't pulled it off) , it handles really remarkably well for a walnut clad cruise liner.
If you are ever around Reading I am happy to take you out for a spin!