Hi
It really depends on what quality your extension cable is.
The RGB coax'es in the extension cable should be of low capacitance, as to not reduce the bandwidth.
The reason you see thick cables on good quality monitors.
In practical terms, I have made an extension of 8m, using 3 external 75r coax cables to carry the RGB signals, and normal cable for the rest.
With the newer type of monitors, there might be a problem with the ID pins, as capacitance there, might cause corupt data. (probably I2C),
sda