Man what an awesome car
I'm surprised more AJ6s haven't found their way into Series XJs. They're by far a better engine than the venerable XK, although the aftermarket's nowhere near as strong.
If the 2.8 springs don't do the job then I've used coilsprings.co.uk before to make custom springs. They've got a big database of stock spring rates so you can tell them 'I'd like front springs for a Series 1 XJ6, but 30mm lower and 10% softer' and they'll make it for you. If you can work out how much weight you've lost I expect you could work out what spring you need.
There's a security warning as you enter their site, but I expect that's them not being amazingly savvy with website design. I've ordered from them before with no issues, but if you're concerned then I expect a phone call would sort an order.