So if a disc hits the ground at 300 feet on level ground, that same throw would hit at 270 feet if the ground sloped upwards 10 feet and 330 feet if there was a 10 foot drop in elevation?
That sounds about right to me.
The 3-1 rule is probably as good as anything out there, which isn't saying much. One of the many problems regarding slope is highlighted by the way you asked your question. If you throw "the same throw" three times with level ground, downslope and upslope (where each slope falls or rises 10 feet vertically per 300 feet of distance), then you probably won't get the 3-1 outcome.
If you are throwing 300 feet over level ground, and lets say its a laser that just glides along at the height that it was released until it fades out, then on the upslope, it will hit the upwardly sloping ground before it travels 200 feet. On the down slope, it will start fading out at the same distance (because it is the same throw) but have an additional 10 vertical feet to fall before it touches the ground. Will you get an extra 30', maybe, maybe not. When fading out, it won't be traveling directly away from you.
However, if you throw upwards on the upslope and downwards on the down slope, you will probably lose more than 30 feet due to the upwardness of the throw on the upslope and gain more than 30 due to the downwardness of the throw on the downslope, especially on steeper slopes.
And the problem with weight is that yours is a purely theoretical question. This is because in practice, no two discs are going to mold up exactly the same. A very overstable version of the same mold is going to get much less distance from the same effort as a more understable version of that mold even if they weigh exactly the same amount. And, regardless of your experience level, no one is going to be able to repeatably and reliably put the exact same effort into successive throws. There are just so many other variables that one ounce of weight difference just isn't going to matter between two discs of the same mold.
Others have mentioned this problem in one way or another and they are pretty much all correct. Humidity, wind, air pressure, effort, launch angle, nose angle (tilt), spin rate, mold, PLH, dome, warp, hyzer angle (roll), weight, dye color (just kidding) - there are just too many factors to consider and most have a much larger effect on the throw than minor differences in disc weight.
As a theoretical problem, I suspect that the answer to your question lies in a problem similar to external ballistics. Assuming all other variables are equal for two discs, then the weight should change the initial velocity and rate at which that velocity (and lift) decreases due to drag. The decay of the spin rate will vary between the two discs and fade will start earlier or later. Maybe a physicist will chime in and calculate all that for you. But as others have said, you'll never be able to tell the difference in real life.
