When loading in a VCD and using the "time based reporting" option to determine peak power, I find that the peak power reported by XPower is unrealistically high, possibly up in the hundreds of watts.
The default timeslice used by XPower when evaluating the simulation is too small. It defaults to 1pS resolution, which is far too small a value for power analysis. This results in overestimation of internal frequencies. In a future release of the design tools, this value will be changed. Until that time, this can be worked around by launching Xpower from the command line and specifying the resolution using the -tb switch.
xpwr mydesign.ncd -s mysimulation.vcd -tb 10ns