I finally got around to making this. After I converted the recipe to all grain, I decided it wasn't going to work at all. The dark extract they used would have too much roastiness for the root beer extract to cover up. I used this instead:
4 lb Munich
5 lb 2 row
1 lb C-20
1 lb lactose
enough hops at 60 min. to reach 15 ibu
When I racked to secondary I added 6 grams of sweetener and the root beer extract. Tasted it a week later, the root beer flavor was weak and it wasn't nearly sweet enough. I then added a second pack of root beer extract and 24 grams of sweetener. Tried it a few days later and it seems pretty good. It's carbonating now and I'm going to tap it this weekend. I'll let you know how it turns out.