Depends what you mean "faster speed". If you mean to the internet then it is unlikely unless your wireless is very very bad. The internet speed you purchase is generally much less than your router can run. If you mean between machines in your house then yes ethernet cable does a much better job because the connection is not shared like wireless. The more you move from wireless to etherent leaves more wireless bandwidth for other machines.
Not sure what you are asking? You plug in the ethernet cable and it continues to use the wireless ? Most machines by default will prefer a ethernet connection to a wireless when both are active at the same time. You need to go to network preferences for both devices and check the settings. Worst case just disable the wireless one and it will only have the ethernet to use.