Before you even begin to talk about the epistar led's, learn about drivers.
The non-led components these "discount" light manufacturers use are the cheapest they can get away with, and their designs compound your problems.
So why does 1kw worth of LED's pull only 300 watts at the wall? It's the drivers. They are likely using very inexpensive, low capacity constant current drivers. There is a difference between a $100 meanwell driver and a $12 no-name/label driver.
So if they only use drivers capable of 300 watts, that is all you see drawn at the wall. They essentially de-rate the led's.
They also use very thin heat sinks, which leads to severe thermal issues. That is the primary reason so many mars like lights have burned out epistars on them.
So in this case, you get what you pay for at the package level.. But it isn't all the LED's fault (although the epistars are 10 year old tech).