Sounds like a plan. Also, the WOT TPS voltage is typically around 4.5-4.7V (or 90-94% throttle open) from what I've seen. Others may have seen higher.
In short, it's likely an issue with your data logger not interpreting the PCM data correctly.
It doesn't matter what it does, it won't make any more power. You don't make more power with a PCM tune on a 3800. You just play with transmission settings, correct fuel system when you add bigger injectors and that sort of thing.
So is this thing just tricking his PCM by sending different frequency to his MAP and MAF? Via some sort of resistor inside the module he spliced in?
This is exactly what it does. They work fine on the 3800 Series I, just not on anything with ODBII. Although the JET modules I remember just snapped in between your PCM connectors to modify the signals, no splicing.