Stencil Buffer only

I’ve been testing how many pixels per clock various video cards are really capable of. This has gone well and came close to what they claim. Now want to test how many stencil pixels can be writen independent of color fragments. I thought just masking out color buffer or setting drawbuffer to GL_NONE would cause hardware to go nearly infinently fast (so I could isolate the stencil), but that doesn’t seem to be the case. So does anyone know of a good way to either blast things directly into the stencil buffer or truly disable the color buffer?