It would not be a good idea to run a chip at 2.5 watts, a bit below 2 watts is a lot wiser, when a usb slot variance can easily dip below 2.5 watts.
Also the chip alone is not the only thing which will be drawing power, you have to take that into account, hence why the design should be below 2watts.
Spec says 500mA, so you can draw 500mA. There is no such thing as "slot variance"; you are making that up...
You would be a pretty bad designer if you needed 0.5W or more to merely power ancillary logic. At most there will be a ~5-10% loss due to the 5V->Vcore power conversion (I doubt the ASIC will run on 5V). The rest (LED) should literally need 0.05W or less. Remember there is no active cooling (it's a coffee warmer). So we are talking about 2.3-2.4W available to the chip.