Given the size of your smoker and a 1500w burner, I think your burner is old or some how not giving off the btu’s of a 1500w burner. However, adding a second burner will get you well within hot smoke temps.
Most griddles are thermostatically controlled for surface temp, not power. The dial isn't an indicator of current or power. It may well be capable of 1500W, but you might need to put a sheet of ice on it before it starts pouring out that much power.
The other thing to remember is that smoking wood releases some energy too, albeit not as much as full combustion and burning. Put in two 800watt hot plates and you may only get to 150F. But put a couple of pans on those plates with wood chips or pellets in them and you'll get both smoke and added temp. This is why cold-smoker folks make an effort to separate the smoke generator from the smoke house and pipe the smoke in.