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The codec used IS critical. If the codecs you have utilize DXVA (and I suspect they do NOT) then acceleration should be mostly automatic.
The codecs that we know are accelerated on US15w are available from Cyberlink with PowerDVD (these are as fully DXVA accelerated as possible) but to get PowerDVD to use acceleration properly you need to ask for the OEM version optimized for US15w and Atom. Hopefully they will be able to help you with getting that version.