I have switched from wifi to wired. I experienced a lot of communication errors on wifi due to the close proximity of other wifi routers and devices to my wii u. Since I wired up, I have no issues at all on my side. Gameplay is so much smoother for splatoon and other games like smash and MK8. By switching to wired, my online experience has improved tremendously.
Lag on the other hand remains the same. I still experience a fair amount of lag. I know its very frustrating in matches where you can never really splat anyone but get splatted all the time. I use .96 Gal but keep getting splattered by almost every other weapon out there in head on firefights in laggy matches.
You will not lose anything by going wired but maybe 20 bucks or so. So until region play becomes a reality, laggers probably make up 30% of A+ now:D If you encounter lag, just rejoin the next group.