I got 11spd on a complete bike I bought and 10spd on my training bike. In all honesty, as most other people have said, there is no gain at all in 11 speed. It would never be a deal breaker for me and I'd much rather have a nice wheelset than 1 more cog, bare in mind it limits your second hand wheel choice. You have too remember that there were pros on 10speed riding with pros on 11speed and it is still this way in the amateur ranks (at least in the uk). But, if you're dead set on getting 11speed I believe you'll need shifters, rear mech, cassette and chain. Shouldn't matter that you're using 5700 you could upgrade the bits to 5800, 6800 or 9000 as all are intercompatible.