I just went through this with a six point install on a OE seat in a Miata.
As the previous poster said, the sub belts get mounted to the same point as the lap belts. Per the supplier and (I believe) the GCR, the correct routing is up across the seat so that you are sitting on the sub belt when you are in the car. It then comes up past your groin to meet the common reciever for the harness and lap belts.
I have read in various places that the if the sub belt is mounted forward of your waistline you can literally be doing more harm than good, and that the sub belt (in some extreme cases I suppose) can actually pull the harnesses away from your body in an impact, allowing you to slide forward and down.
It would seem to me that a six point (or the coming seven point requirment) would be better than a five point just because of the additional points for loading force in impact, thus requiring all points to carry proportionally less load.