Mario Kart Wii has an entirely different approach to lag though that wouldn't work in a fighting game. Mario Kart Wii uses AI to predict your opponents path during lag. Then when it receives actual coordinates for them it updates accordingly (sometimes causing them to fly to the opposite side of the track). That's why you can throw a Green shell at someone and they won't take the hit at all even though it travels right through them. They weren't actually there, but 4 feet to the left of that spot. You'll notice a lot of bouncing around if you do a worldwide match when compared to regional. The games not actually lagless but goes about it in a different, less noticeable way.
Brawl forces the game to lag equally (most of the time) and slows framerate to compensate for lag on all players to keep it even. How fun would it be to take a swing at someone, only to find out they weren't actually there but behind you charging a forward smash. Brawl uses the system that works best for a fighting game. Every game uses a different way to compensate for lag.
I think the adapter helps though personally. I have noticed I haven't had as much input lag as I have had in the past. It's not a very big difference though, so I'd save your money if your trying to save for more important things.