While I can not give a definite answer, I can say that I have run into _big_ problems with this when we tried (ignorantly) to lengthen a VGA cable by adding on another F-M cable. Tried an amplifier and it was not much of an improvement, but wouldn't you know, a new cable the right length fixed it in an instant.
The run is about 60 feet and we added about 5 feet to that in order to move the PJ. The actual PJ is a Benq something or other. It is a mid level 720 resolution and even though it has HDMI, it is a conference room PJ and mostly sees use with spreadsheets, eDrawings and junk like that.
We certainly learned our lesson on the VGA cable deal. Painful, but a well learned lesson for sure. Based on my experience, I would certainly not recomend extending the cable if it is already there and you just need a little more; it sure messed us up.