I could see 500 mbs taking a while on a congested service with millions trying to connect at once.
How many consoles are you expecting them to ship on day one? These servers aren't going to get slammed nearly as much as people think. The XB1 has it the worst off on launch with it being launched in the NA/UK/EU on the same day, and even then it won't be that bad considering the launch will be staggered by time zone, and when people actually get their console on day one since so many are ordering online. All of this is on top of the fact that there are going to be a pretty limited number of units. This will be nothing like the launch of an MMO or other always online game.
I have a very hard time believing that any server issues at launch of either console will be due to server load.