Curious. Does it make that much of a difference? I've always played games online through both ethernet and wireless depending on which device I'm on and never had a big issue. I was getting a massive delay in my attacks, a good few seconds behind. Surely a cable isn't gonna help with that?
I should note I've only had time to try one game so far.
I can't speak specifically for Smash yet as I haven't cracked my copy open yet, but it'll keep YOUR connection stable. It's a must on PS360 fighting games IMO and it removes all lag spikes I have while playing DotA. I consider it a necessity but your mileage may vary. At the very least it'll give you a consistent level of performance.
It won't stop lag if your opponent has a shitty connection unfortunately. That's on them.