If you like the look of a spare wheel on the boot lid, do it.
However:
To move weight from low in the boot to high on the boot lid is the about the worse thing you could do to upset the handling of a car. I'd say if you don't notice the difference to the dynamics of your car, or don't care about this change, you don't need uprated springs and anti-roll bars etc.
A major part of modifying a car to improve its dynamic handling, is to lower the CG and reduce weight fore and aft of the axles. This is what Cobraboy has done by removing the weight behind the rear axle. The dynamics of a car is very dependent on the height of the CG at the rear and front, together with rear suspension geometry different roll centre heights, front to rear. These roll centre heights will be designed to work with the CG heights, to try and produce a car that handles predictably.
The roll centre height of the rear suspension on the P6 is somewhere around the height of the rear axle diff, approx the same height of the spare wheel lying flat on the boot floor. Therefore removing the spare will should not reduce roll very much, but as Cobraboy states reduces the weight behind the axle and makes the car quicker to turn and will make it quicker to stop turning if it has started to oversteer. (quicker handling). Rising the wheel to the boot lid, will make the car roll more. With the car trying to roll more at the rear, than it was. (It's trying to fall over more at the rear than the front) This is just bad. It hurts my head just thinking about this. Its why I don't like to drive those modern 4x4s.
For me the best place for the spare wheel is strapped behind the passengers seat. That way I nip around the 4x4 at roundabouts.
Don't get me wrong here, if you like the look, go for it, it's not for me. I'm from the sporting saloon side of the fence..... I seem to have strayed, I'll go back over to my side. I'm blaming Cobraboy for this, he brought me here. Sorry.....
Gav.