I just looked on the Ryobi site, and couldn't find a 6" polisher or sander. I'd say it would probably work if you could get a 6" velcro backing plate for it (might even come with one). The Porter Cable polisher that we talk about is variable speed and goes 2500 - 6000 opm.
A couple other factors are the "stroke" of the orbit. Finishing sanders vibrate fast, but not very far. A good polisher will move maybe 3/8" (thats about what the PC does, I'd say). Another is the strength of the motor. Some bog down under pressure, others don't. I haven't found the PC to bog down much...some, but not a lot.
Another is durability of the tool...I've felt a few other cheap polishers...black and decker I think it was, and while it may have worked, it felt kinda cheap. I don't know how long it would have held up, or how much it would bog down. The PC feels like a quality tool...but then again cheap is just fine in some instances.
side note: my boss at work had a ryobi weed whacker that he brought in for me to use (clean up around the building). After maybe a total of 5 hours over 2 days, the thing broke a connecting rod! I'm not saying Ryobi is bad, or a more expensive whacker would have necessarily lasted longer, but thats what happened to me. (He brought the machine from home...he'd had it for a few years I guess...none the less, I felt kinda bad)
Kevin