Marmalade offers an excellent Native Extension framework
http://www.madewithmarmalade.com/marmalade/features/extensions-development-kit
WHich ultimately means that you can jump directly to a native implementation of any particular feature. You still keep the core benefits of cross platform development for your main app.
Also on android because marmalade makes use of the android NDK your c++ code for processing data will be running faster than the corresponding android Java code.
Im making games with Marmalade and the extensions and native code speed make me extremely confident of being able to deliver at least as well as a 'native' app.
