1.5 is brutal for FPS and most games in general. Too much bandwidth required for xbox360 generation games.((sadly some people dont have the option for higher speeds)).
12 will honestly be plenty. There WILL be some times of lag but wont be nearly as bad. A video will now take 30 seconds for example to load as opposed to 3 minutes. Which means even if it lags it wont be extended.
If you want the best experience purchase a router ((or see if your router already has)) QOS settings. This will allow you to prioritize the XBOX traffic and make sure that your youtube isnt cutting in line and screwing up the matches.
oh and be sure the xbox has an open NAT or DMZ set up for it. You can google those terms and find great step by steps for setting it up. That will help minimize connection issues on the XB.