And by definitive I mean theoretical but first a little background. I've been doing some research into swapping out the suspension components of the LS for those from an '03-05 Jaguar S-Type. This would give me an improved handling, lower stance, and the brembo brakes. What I have found leads me to believe it's still possible, but requires some pretty big conversions.
Starting in 2003 Jaguar converted the lower control arm to be a two-piece articulated design. There is the main rear piece of the LCA which supports the weight of the car and then a second arm attached by a bolt at an angle. This gave them increased suspension geometry control over the full suspension travel as well as an almost 10% increase in the rigidity of the suspension.
The down side of this means that the front k-member would have to be changed out to do the conversion. There is also no guarantee that the k-member bolts up directly, or that the UCA is positioned in the right spot to maintain geometry. I feel fairly confident that it would all bolt up and work properly though.
So, here's what it would take to do an 03-05 STR suspension conversion on an LS:
Front K-Member
Front Lower Control Arms (both pieces, both sides)
Front Upper Control Arms (both sides)
Knuckles for all 4 corners
Brake Calipers for all 4 corners
E-Brake Calipers for the rear (both sides)
Brake Caliper Brackets for all 4 corners
Brake Rotors for all 4 corners
Brake pads for all 4 corners
e-brake cables for rears
STR Springs, shocks, and everything in between for all 4 corners
Front and Rear sway bars
A keg of beer
lots of cussing
misc hardware
So yeah, that's all. Probably $3K worth of parts, not counting labor.
Here's a computer rendering of the front k-member. You can see how the forward small arm is mounted at a different angle. This is what helps stiffen the suspension and maintain geometry as it flexes: