a bit late to say but if your dyno operater is really hell bent on giving accurate comparison.. then he could use an accurate infra-red or contact thermometer to read the temperature near the coolant outlet of the head or via diagnostic interface on some of the cars. I do a lot more things to ensure repeatability when tuning, but most is way overkill for just power-runs. coolant temp is the most essential thing for havign a true comparison.
just got back.... had a good afternoon. just want to say thank you to Chris for organising. next time, i will make sure to have sufficient oil and a service