The Cobras I flew in Vietnam were about 1200 lbs over gross weight, and the max altitude for hover was about 6 inches. We made sliding takeoffs everywhere, and gained about 10% more payload in the process.
Most helos now are performance designed around the IGE hover, so that MGW matches IGE hover weight under normal circumstances. This means you can't gain payload without busting MGW, even if you make a rolling TO.
But at altitude, it is possible that you can get an advantage. Be careful, it is quite possible then to find yourself in an embarassing position where the landing is a short squatty one as you fall through, if you forget and try to hover. Of course, you will be saved by jumping out and shouting "VRS got me!"