Will a 2000w Step Down Transformer be enough to safely power a Japanese Oven Toaster?
The Japanese standard is 100 V, whereas in the US it is 115-120 V.  The higher voltage will cause more heating.

Power into a resistor is proportional to the square of the voltage.  Let's say worst case we compare 120 V to 100 V.  The resulting power ratio is (120/100)2 = 1.44, or 44% higher.

If it were just 10% or 20% higher, I'd probably just try it directly.  45% more power is enough I'd worry about over heating.  If this toaster oven has a thermostat, then maybe you can still get away with it.  The heating elements may burn out, and they will get hotter than intended, but they will be on for shorter amounts of time to result in about the same oven temperature.

All that said, why not just get a toaster oven intended for the power voltage you have?  They are relatively cheap, and don't come with the same risk of burning your house down.  Sell the Japanese oven in Japan and get a US oven in the US.  It may even be cheaper overall if you consider the cost and hassle of shipping it.