A weight of about 5kg would probably be ok with the screw in plugs, Tommy Walsh is associated with them, they would screw into just the plasterboard, however I think its dependent on the strength of the rather than the plug. For an LCD I, personally, would not trust the plasterboard, I would try and drill into the wall itself, behind the plasterboard, pop a wall plug into this hole and screw it down. You'd need to be careful not to screw in too far as you would dent the plasterboard. Your ideal fix is to locate studs, but as you have highlighted this is not possible