Microsoft not supporting XNA is no longer relevant in this argument. MonoGame implements XNA and we are actively supporting that platform across a wide variety of distribution channels. I encourage everyone to give the platform a try before lampooning it. The Independent Game Studio channel is highly valuable to Microsoft and it will not go away. Any other cautionary claim is just FUD.
XNA will get you where you want to go. We’ll also get you into the SONY world. You won’t do that with C*+ and native SDKs.
Every platform will have its weaknesses. If you have a performance issue in C*+ and Windows 8 then it may likely be something in the way you are using the graphics hardware.
IT’s very important for everyone to understand the limitations of their hardware before porting a game. Each mobile and desktop device has a wide range of capabilities and all of them suffer some limitations. A very common performance problem in the GL world is alpha blending and full scene multisampling.
If you are already using C++ and cocos2d-x and are just tuning the platform, then I encourage you to continue tuning and push your changes back to the team. I don’t see a lot of PR activity on github for cocos2d-x which is very discouraging. Send your changes! We all benefit from changes that you share upstream with us.
Good luck! Continue the conversation as it helps everyone who is considering the same problem. Also note that your XNA game on Windows Phone 7 will ALSO work on a Windows 8 phone. All of our XNA games are available to both Win 7 and Win 8 phone devices without any problem.
Sorry for the rant. All of us in the cocos2d world enjoy the users of the platform and want to continue to support you in the most productive way. Please help us by using github and contributing back to the effort.