208V vs 240V efficiency numbers aren't commonly published, but the difference is likely to be less than 0.5% or so.

For example, an unspecified HP PSU measured in 2007:



BTW I'm making my measurements with a calibrated Xitron 2503AH (http://www.xitrontech.com/products/general-purpose-power-analyzers/#2503ah) with an external 500A precision shunt for the DC current.
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Nice.  I am jealous!  We may rely on your for accuracy.

Let me know what numbers you find. 

@Philipma, be careful when testing to 2500.  I have not tested these boards beyond 2kW for an extended period of time.  It should be safe, but try not to exceed 130C anywhere on the board.

-Optim