It reduces rotating mass, which should make the car feel a little more athletic / rev easier.
As far as the tension goes, I just felt the tension of the belt before I took it off, and made sure it felt the same when I put it back on. You want it snug so it's not going to come off, but you don't want it so tight that you can't press it down a little bit.
There are issues with other cars (my BMW 3 serries for instance) where replacing the stock pulley causes crank dammage long term.
I hear this is not the case on the Subies. Is it true that the stock pulley does not act as a dampener? Of course it will still act like a dampener to some degree in that it has mass, but if it is just a heavy and evenly weighted pulley then installing a light weight one shouldn't be an issue.
Any educated opinions on the matter? The only thing I have herad so far is "We've sold xxxxx of them and haven't heard of any issues etc"
With the parking brake on and a stopper behind the rear wheel I ended up rolling the car OVER the stopper when trying to break the pulley bolt. You might be able to do it, but I would recommend a foot on the brake.