Well, the line with 4..fxe4 5.Nxe4 dxe4 6.Nxc6 Qg5 is difficult to play OTB, because 1)you have to know a lot of lines 2) it's not easy to play, you have to be a "good" player. 

So, in this line Black has to know the idea with 17..Bxc4 followed by 18..h6 as a "pawn sacrifice" to get the draw. Here is a possible continuation: h6 19. Qe6+ Kc7 20. Bxf6 Rxf6 21. Rxf6 gxf6 22. Qf7+ Kb8 23. Qxf6 Qe3+ where White get nothing to continue. But if White want to continue OTB, Black still have to be precise at this point, so I play the Schliemann on corr without problem but OTB, that's need some courage, skill and preparation! I use it from time to time OTB but that's not a easy weapon to master, respects to Radjabov!

I don't think that line is dangerous for Black. I have played against that in a corr game and I faced no problems. 17.g4 Bxc4! is maybe the best. 18.Qxc4 h6! and now I challenge you to find an advantage for White, I only see a draw by repetition soon, but if you find better I would be interested!