IEC socket rated for 250V at 10A - can it be used for 13A @230v?
No, it cannot be safely used.  The socket does not dissipate any significant power, so it doesn't care about the product of voltage and current.  (It doesn't care about the voltage at all, as long as it's low enough to avoid dielectric breakdown... and that would require thousands of volts.)  

The only important parameter is the current rating, and that's 10 amps for the case you're referring to.  10 amps through the socket will heat it up just as much at 1 volt as it will at 5,000 volts.