I believe your thesis is correct that the gut mains would have a lower DT reading than the Poly mains racquet. However it would be interesting to see if this would be the same if you strung them how most do with the gut being at a higher tension.
Most players who come into my shop asking for one of the hybrids I set up with the following rule.
Gut mains- 4 pounds higher than the poly crosses
Poly mains- 2 pounds lower than the gut crosses
This is in no way scientific but I do so because I am compensating for the length of the string of the mains compared to the crosses. I believe that with these tensions the DT readings for your racquets could be much more similar.
I've never actually used a DT meter and would be interested to hear your findings if you actually go through with it.