Jim, I don't doubt that what you say is true as far as the output. However, there may be more at play than just that fact that one is mono and one is poly. You have two different manufacturers, different IV curves, different temperature coefficients, probably different panel ages. Panels also have a power tolerance, typically ±3 to ±5% even from the same manufacturer or batch.
It would be nice to see some real documentation on this widely believed but unverified topic. Some websites actually say that mono is better in low light and does slightly better than poly in higher heat. Other sites say that poly is better in low light and does better in higher heat. Whom do you believe? I would think that the actual specs for a panel under consideration would be more important than whether it is poly or mono.
