Have you done any side by side testing against a plastic "Igloo" type cooler?

Yes.

We tested side-by-side with a standard 10 Gal "Igloo" type cooler.

The testing showed the plastic cooler lost heat faster than the InfuSsion Mash Tun.

You can expect the plastic cooler to lose an extra 5-7 F over a typical 60 min mash depending on a variety of factors.