I would start to worry around 20 kg. Probably worth having 2 screws then. I fitted one the other day must have been 25-30 kgs, it was supplied with a French cleat fixed with 4 screws. Total overkill because you would have trouble getting it off the wall with a crow bar.
Usually a 6mm rawlplug and 5.5mm screw or similar. 8's are kind of overkill and 7.5 kgs is not a lot of weight.