[casioqv]

If we imagine your described ideal dual diagonal bar setup, the outside strut diagonal will be compressed in a turn, and the inside diagonal bar would be under tension...

So if we are using a single bar due to spacial limitations, having it under tension would seem to better control deflection on the inside, and having it under compression would seem to better control deflection on the outside. (edit: note that these are in direct conflict, so we have to decide which side is more important to stabilize!)

Now, consider that a Boxster basically unloads the inside rear wheel at maximum cornering, so all of the force is really on the outside wheel. Given this situation, wouldn't a compression pre-load be what you want - by prioritizing the correct geometry for the more heavily loaded side?

edit: Also, since the forces are higher on the outside and we we would expect the deflection here to be much greater, in a turn the bottom of the suspension sides will get closer together (compress together), because only the outside suspension has enough force to really move a lot. This suggests that a bar pre-loaded with tension would actually exasperate the suspension flex issue, causing further increased flex!