The other day Meagan and I were out climbing near Buena Vista, Colorado. We squeezed 8 pitches into a seemingly typical showery spring day. For a variety of reasons, we ended up doing some top-roping on real skinny rope (7.7mm). This is admittedly aggressive, but we made sure that we weren't running it over any sharp or abrasive edges. We also made sure that we could lower smoothly on the inherently lower friction cord. In order to increase the belayer's holding power, we used a regular belay device, and added additional friction. A second carabiner between belay loop and rope is the first and easiest step. If that is still not enough grab, one can create a woven carabiner brake in addition to the belay device. See the photo below for the detail.