Using a 2000W step down converter for a Neo Geo (or an alarm clock too for that matter) is running within specifications. You are mixing it up with changing the AC-Adapter for the system itself and running that one out of spec can cause trouble.
The step down converter mimics an outlet and trust me, any outlet is able to deliver more than 2000W or you would pop a fuse every time you use a hair dryer or vacuum cleaner.
This is not about sledge hammers and nuts, this is more like buying a new table and figuring out how to bring it home. Either you rent a volvo or you rent a truck. You will still get the table home, but the truck just isn't needed unless you buy 10 tables which is more than the volvo can take. The truck is bulky, unnecessary and expensive but well yeah, you do get the table home.
Watts and Amps are about how much load you can put on the step down converter. You can have a million watts step down converter to run a gameboy. The gameboy takes what it needs and leaves the rest. However, if the voltage is incorrect you will damage your equipment.
Keep the Watts and Amps equal to or above its requirements and you will be perfectly safe. The voltage however HAS TO BE correct or you will kill your system.
