Firstly I would take it to another RR! it is possible for RR's to underread, but you did mention that Golf R's have been seeing 300+
Could be possible that factory fitted a 265 ecu you your car by accident or its got a leak somewhere.
Had mine done recently when getting it mapped. The stock power was 271 and torque 245lb/ft. Couldn't get an accurate after test due to wheelslip, best we saw was 309bhp at around 5300rpm. But dyno would suggest around 330 max, torque is up 40 - 50lb/ft I would guess. Will know for sure in a few weeks time, though being a different set of rollers on a different day means it won't be comparable