"There's simply no Open XML product on the market yet, to compare performance," Marcich said. "ODF is supported and implemented not just by OpenOffice, but by multiple applications including StarOffice, IBM Workplace, KOffice, Abiword/Gnumeric and Google Writely. All these applications have different performance behaviors."
Of course, there's no reason why both standards can't be accepted and implemented. However, it's unlikely that MS would be happy with dual standards.