Did this today and it's definitelly a noticeable difference. I used Phillips OEM replacement H9 bulbs from Amazon. The Subaru OEM H11 bulbs are also Phillips and look almost exactly identical except for the color of the plastic and the tip of the H11s is painted over.
I had the H9's in and decided to take a photo comparison and when I put the H11s back in I thought I did somehting wrong because it looked so dim, but in reality the H9 bulbs are just significantly whiter and brighter.
so far it's the best $20 I spent on the car. Here is a comparo pic:
H9s on top
It's a little hard to see the difference in the photo, but it was an obvious difference.