Redoing the client in a different programming language

I came to the conclusion that developing the client as a WinJS-only application was a bad idea. The canvas implementation of Metro apps is too slow for what I want to do and I am essentially closing out all people except for the Windows 8 early adopters.

So which programming language to use?

Keeping the client in WinJS:

• + profit from the Windows 8 early adopters
• + collect Windows 8 experience
• - performance is horrible
• - badly documented API
• - Testing and development in a VM

 Platforms to dismiss completely: Everything but Windows 8

Client in Javascript:

• + very accessible, because it requires no plugins and no download
• + keep much of the existing code
• + GUI in HTML
• + users behind proxy servers can play (websocket / AJAX go through HTML)
• - users behind proxy servers can't be IP-banned efficiently
• - it's a horrible language for larger applications
• - annoying differences between browsers
• - WebSocket is currently disabled or unimplemented in most browsers. Workarounds exist (AJAX long polling), but the performance is unsatisfying.

 Platforms to dismiss completely: only Windows XP with IE8, it would even runs on most smartphones.

Client as Java Applet:

• + Almost as accessible as JS
• + Runs on a lot of platforms
• + share code between client and server
• + powerful library 
•  + data exchange via serialized objects is possible (but turned out to be 4x as bandwidth-intense as JSON for some of my test cases)
• + applet allows integration into browsers
• - But no communication with the website, because Java DOM isn't supported well enough
• - Java plugin required
• - Conspicuous Do-you-trust-this-applet click required unless I do without some crucial features

 Platforms to dismiss completely: Android and iOS

Client in C++:

• + Very good performance
• + platform-independent development is possible, but a lot harder than in JS and Java
•  - development will take longer due to low level stuff I have to deal with
• - Binary download and installation required

Platforms to dismiss completely: Mac, most Linux distributions (sorry, but I can't create packages for all of them), Android, iOS