There are drywall installations and then there is the OP's drywall install first off. If what he has told us that his joists are 5 to 6 feet apart then by no means should he consider hanging 10 pounds of equipment midway between the joists. In fact I'm surprised if the drywall isn't sagging under its own weight at that span.
Now for the case of normal 16 inch on center construction and molly anchors of the type shown. There are many variations on drywall applications it comes in different thickness and is attached in different manners some is nailed some screwed and some glued in conjunction with screws or nails. Some upscale jobs use 2 layers and some is plaster coated some painted. If you weren't around during construction you could even be dealing with full plaster.
Drywall is basically two layers of thick paper with gypsum filling the gap between and as posted above it is a fairly strong material but that strength comes at a cost and that cost is its extremely brittle once the paper surface is compromised its strength in the area around the cut is greatly lessened. That's why when you mount something into it and give it the tug test as mentioned above it feels very secure. Its not like wood or metal where it will bend and flex before it yields, with drywall its there or its not.
Will 4 anchors hold a projector mount in place with say 10 pounds of weight pulling on it in half inch drywall that has been properly applied? The answer is yes. Is it the best way to recommend doing it? I would have to say no.
Here are a few things that can happen in drywall. If you get unlucky and happen to put two of your four anchors into the joint line of two sheets and they fail the other two are coming down. the gypsum around the drilled hole gets fractured when drilling and tightening the anchors and the paper facing on the back side can get scored depending on the type of anchor used. Etc
Its not a awful method of hanging something people have been hanging plants and swag lights off these things for 50 years now. And most have held fine. But in the case of a very expensive piece of electronics that's warranty isn't going to cover it for falling why not take the little extra effort and secure it into real building structure. If cables are going to be run thru the ceiling you will have to get above it or make a opening anyway. Drywall is not that hard to patch in and you will have the peace of mind knowing its really 10 times over kill.
If you want to go with the anchor method and haven't used them before below is a good read on the types and how to best install them.http://www.naturalhandyman.com/iip/i...infanchor.shtm