On an interesting note, I have never been able to get my overloads to engage without overloading the truck.
And the OE load rating is not affected by any modifications in any event; my state restricts trailering to "manufacturer's rated capacity." Like the law in every state in which I'm aware, modifications neither increase, nor decrease, the ratings the factory gives. The GVWR and other ratings are on the door or door jam.
The reason your overloads didn't contact is their tuned to the trucks RAWR...not the truck mfg GVWR or its GVWR payload.