for starters, whether or not weight affects top speed is dependant on several variables, the most important, I would think, would be whether or not you were able to GET to the redline at both weights.... if so, there is no difference in top speed, only in acceleration.
I assume, perhaps incorrectly, that this rider was able to redline his bike by his speaking of it not making any more power no matter how long he stayed on the throttle and his refference to this high of a speed (dusting the YZ's). That being the case, it would go faster with a bigger CS sprocket on it.
Perhaps his bike is poorly set up and won't redline his weight though... in which case a larger CS sprocket may actually make the bike slower because it can't get up to the same RPM with his weight. If that's the case, I doubt even more that he got it up to 80mph, meaning his speedo is further off than I suspected.
I suggest calibrating your speedo, because I think those speeds on your setup are likely impossible without destroying your motor. The speedo won't help you much if it is off by a substantial amount, and it sounds like it is.
And then there is the question. Why would you run a 12 CS on a bike you use on the road? Even if it will do 80 now, it will do it easier with a 13 or better yet 14 on it. (given the aforementioned faulty bike setup is not to blame)
Oh, and not only is 80mph on knobbies on tar scary, it can be downright dangerous at times, ride safely when youre clocking your bike. There could be little worse than injuring yourself while testing your bike.
Take care, and check that speedo for us so we aren't all wondering what the heck we are doing wrong with our bikes.