The speed rating actually has more to do with tire construction when you're shopping around, not the actual speed you'll be travelling at. Generally speaking, almost all tires within a given speed rating will be constructed in a certain manner, which in turn affects how the tire performs on the road, even at very low speeds. The performance factors that could be affected by a change in speed rating include handling (especially emergency manoeuvers) and braking.
That said, you may be able to find a lower speed rated tire that performs like your originals in that regard, but there will be no way of knowing that ahead of time.
Other times you may downgrade tire speed ratings, and this is always strongly cautioned with lots of butt covering: driving style is very sedate (slower than grandpa on a Sunday morning), used as a winter tire where lower speeds are expected (and therefore less need to push the tire to handling extremes), but with the expectation and understanding that you are compromising things such as braking and losing road feel. You could potentially be compromising your own criterias D and G.
So unless your needs fits into the above two scenarios (which I don't think it does), then I highly recommend against going to a lower speed rating.
From your needs, maybe consider the Falken ZIEX 512. These are very well regarded, and competively priced, and would be a good match to the Forester.
