not exactly what you'd think
Depending on design, the 6th gear is usually a second overdrive, or used to get closer gear ratios. As a second overdrive, it drops revs and gets beter milage on the highway (or ultimately higher top end, hence the MKIV's 200mph+ ability). As closer ratios you stay in your powerband easier.
The V160 is a rediculously stout tranny to take the abuse I've seen it take. At those levels though, if it DOES break it's driver error (like missing a shift).