I don't have a Simplicity but I do have two of John's exchangers added on my machine. If a guy has a large Kohler (has hotter exhaust than others) he will be very happy at 06 flow. I'm using it without turning on my propane burner on most resi work with 15 flow.
It is not all about how hot it gets. John uses stainless tubes and shell for durability. Coiled copper tubing will transfer heat better but I never had one that lasted as an exhaust exchanger.