I personally would think the total opposite - Running down the highway @ 65-75mph with a slight load on the 6.7L would generate a decent amount of heat - I know my DPF temps are around 650-725*F at that speed. (According to my VeePeek & OBD Fusion App) Gauge.

Once my gauge shows - I'm in Regen - These Temps get up to 1165 - 1200*F @ Highway / Interstate speeds (Over 40 mph).

I've notice mine once below 40 mph will step out of Regen according to the gauge & my DPF temps will drop back down to normal.

Parked & @ idle these same temps drop well down below 500*F.

The above rambling is only my observation of my OBD Fusion App on my iPhone.

Anyway - That's awesome; his solution helped Great News!

Thanks for sharing your experience.