Yup, you lose 1 disk out of however many disks you have in the raid set. As you add disks, you gain speed because you have more spindles, and both streaming reads / writes get faster. But rewrites get dramatically slower as you add disks.

So it really depends on your usage pattern of whether more disks are better or worse for you.

Of course, there are many other variables to think about if you were really pushing for performance.