Hope your trip went well.

I tried the bank fueling bit and no change? Still had difference on 1 and 2... I'll give it a shot again tomorrow. Also asked John at TC to verify it's purpose.

As you found TunerCat will do the Import Calibration of Engine/trans or... which others call a segment swap. It will also do it table by table if the table is open. The more I use it the more I like it.

The EGR helping / hurting mileage has been a back and forth issue for years. Claims on both sides by respected individuals lead me to believe there are more than simple variables involved. I look at cam overlap and exhaust configuration to decide how much is needed. Decent overlap and mufflers indicate EGR already present at part throttle without opening valve so why add more?
That is the first time I've ever heard a theory on why! Makes perfect sense as well.

I just can't pull the TB off right now to check EGR/diffuser issue you asked about, can't chance the vehicle being down. Have no idea when I may have to leave town and bring wife to OR for surgery... The Suburban just has to many issues for a trip...