Putting a 185 on an 8 inch rim is a different proposition though. It would be REALLY hard to get that tire to seat on that rim. Anyone else who has had a crappy tire mounting job in their lifetime like I have should be able to back me up on this. The beads of the tire will be nowhere near the rim, and they won't be even close to straight up and down.
I don't know this for sure, but it seems apparent that the beads of a tire that is properly sized for the rim would be less likely to have problems because when given a side load the tire can flex a lot. If the tire is already flexed because you have a 6" tire on an 8" rim, there could be issues. Again, never seen it myself, but it always worried me, especially with the 4000lb Caddies riding on 185/50/13 on an 8" rim, vato style.
I agree with Pete, if it doesn't make the car faster, don't bother.
Jon