Italian meringue: how to adjust sugar?
Generally speaking, I think experimentation is the way to go. There are a lot of recipes for Italian meringue (and Italian meringue frostings) you can try to see what effect the different sugar content has. Reducing the sugar can definitely reduce the stability, but stability is a somewhat subjective term. That's why I recommend experimentation. It's hard to say what would be an acceptable reduction for your personal preference/needs.

That said, the amount of water you use to initially make the syrup should make no real difference. By the time you add it to the egg whites, the sugar's at softball stage, which means that all the water you added is gone. The vapor you still see is actually evaporating from within the sugar itself. So don't measure the water. You just need to pay attention to the amount of sugar and egg whites, same as with another meringue. The only complicating factor here is the heat from the sugar syrup. If you reduce the amount of the sugar syrup, you will reduce the overall temperature your egg whites get heated to, and that could reduce the stability.

I have not experimented extensively with pure meringues, but rather with meringue-based buttercreams, and that is a different situation. However, I have managed to make a successful meringue base using sugar syrup heated up to hardball stage (it was an accident, but it worked.) If you did make less sugar syrup but heated it to a higher temperature, that might help maintain stability, but you could risk some sugar crystals, depending on how quickly the syrup tries to cool as you pour it into your mixer. You could also risk some scrambled eggs. I doubt that, but it could happen. 

Another option would be to make the same amount of a caramel syrup. Cook your sugar until it's taken on a nice deep caramel color, remove from the heat, and avoid the giant cloud of steam as you pour in some new water. Then cook that less-sweet caramel syrup until it's softball stage and proceed as usual. Obviously, that will impart a caramel flavor and color. It's up to you whether that's a bad thing. On the plus-side, that caramel syrup is great for coffee. And ice cream. Seriously, you should do that even if you don't want to make a meringue with it.