Not. Sagging voltage is a symptom of an oversubscribed circuit. This can be the result of a very long wire run, undersized wires, other devices on the same circuit, or some combination of those. In any case, a Variac cannot help. It will only make it slightly worse, since it, too, will use some power, adding to the load on the circuit.
First thing to do is make sure that the Bullet is the only thing actually running on that circuit. Note that the same circuit may serve up to 5 or 6 outlets. It’s also possible (but probably not to code - I’m not an electrician, either) for there to be lighting fixtures on that circuit.
Are you certain that your house has the full 120V available? It could be that 119V is all you’re getting from the service line.
Also, check the wattage reported by the Kill A Watt. (Alternatively, you can measure the current in Amps and multiply by the voltage to get Watts.) I think it should be just a bit over 1300W, if things are going well.