Answer
Not at all, rs232 is one of the oldest interface still used (it dates back to 1969!).
in the most "advanced" variant it can carry a maximum of 215 000 bits per second, which is way too low for a decent video signal.
to give you a comparison point, hdmi - which is the connector of choice for transmitting digital video signal - has a data rate of up to 5 000 000 000 bits per second.
in the case of the sky digibox, the rs232 port is here just to allow an upgrade of the internal software, to perform diagnosis without opening up the box, or to connect a gamepad.
