Even on a really fast connection, it flatlines out at cable speeds for browsing, for centralized download I usually get about 500-700k depending on the host even though I have a 15M download. However for P2P, the situation is similar in the case of small numbers of uploaders, but if you have 5 files being downloaded at the same time from 5 sources on each file, the bandwith really adds up and my connection handles that kind of strain much better than it did with cable. Also, for gaming faster is always better.
Unless you're gaming online in a very latency-sensitive game or downloading a lot of large P2P files or from places with really fast servers and connections, you'll see it flatten off at about 2 Mbps download, and DSL is just a tad below that in my opinion. At least until more places upgrade their hardware to keep up with the even faster users.
On Wifi you have a 54 Mbps connection to a point that has a 1.5 or 3 Mbps connection, so you're bottlenecked by your ISP. Your computer detects the 54 Mbps because it is a part of that connection, it won't remotely sense the slowest part of the network chain.