We can ensure that the LAN side and up to a point on the WAN side is working efficiently for gaming, anything beyond that, there is too many variables and too many people that don't use the same set up or have good networking management as we do so, ya, there will be those gaming on sub standard ISP service lines, ISP modems, ISP wireless, non-DLink routers or configurations that are not correctly set up for networking and gaming.
I always run into problems even with my set up. The one thing to do is that if you have a group of people that you alway play with, talk with them and teach them about setting up there systems to be the best for each system that way you know at least the group of people you play with, you systems are running well and any problems seen will be considered a gaming issue or your competitors networking issues.
Enjoy.