Does heavy cream "go further" than light cream or milk in soup
For soups like this also consider evaporated milk.  It contains only about 8% milkfat, so it has significantly less fat even than half and half, which contains between 10% and 18% (cream of course has more).  Evaporated milk also comes in a can, so its easy to keep handy.  Texture-wise, because its so much thicker than regular milk, it makes an excellent replacement for cream.  As far as taste goes, I think in soups and other strongly flavored dishes where the cream is present only to provide creaminess you won't notice a difference.  In something more delicate or where the cream itself is a flavor component you may need to do a little experimenting, but generally I would say you could substitute it 1:1 for cream. 

For your original question: "will the flavor be the same if I use, say, a 1:2 substitution for heavy to light cream. That is, can I use half as much and still get the same flavor?" I think it would depend on the quantity.  Generally, the larger the quantity called for in the recipe, the less heavy cream you might need to achieve the same result.  So, if the recipe calls for 1-2 tablespoons, I would bet that you would want to use the full amount.  However, in something that calls for a cup of light cream, you may be able to get away with 1/2-3/4 cup of heavy cream, depending on the dish and your taste.