People think that super fast internet speeds mean a whole lot, they really don't. A run of the mill 7/1 DSL connection hardwired to the consoles ethernet port should suffice if the netcode is designed well. Your download speeds won't really improve the performance a whole lot, the most important factor is upstream bandwith. People also forget that pings really affect performance. You can have a monster connection and if your connection to the other peer or server has lots of latency, then your connection means crap.
If you are connected wirelessly then performance will drop regardless of internet speed tier. There are latency issues and the obvious limitations of not having a wired connection. Lots of data gets "lost in translation" causing performance issues. Also as someone else mentioned, with Cable providers, you have a dynamic IP adress and are in what is called a Node. If your Node is congested, then speeds will fluctuate and latencywill fluctuate. The best approach to fixing that is by changing your MAC adress and getting on to another range. With DSL, Your IP is static and bandwith is dedicated, no shared node.
One thing that would make life easier for online players of Injustice is if it had a feature where it allows you to filter the bad ping connections. For example, only display players with 30MS ping or less regardless of region. That would weed out the bad pings and connect you to people that are within closer proximity or have a great connection to you.