The only thing behind the flat pieces on the M5 is the tow hook on the one side......
Notice on the M5 between the ducts under the fog lights there is a recessed channel running across the bumper between them.....this is NOT present on the M540....interesting
Also Geoff important for your swap is the temp sensor goes behind the blank duct on the pass side.
I would actually think the parts are interchangeable so if they don't cost too much....probably worth a shot!