Brah, mine had choke rust like that too, but on the rear axle and parts of the frame when it was brand new.
I was lucky that I checked the underneath a day after we got it. Dealer fixed it, but not before saying how it was "normal", blah blah.
When we got it, they offered to "electronically" rust proof the truck. Basically, it runs an electrical current through the frame, etc. and they put a sacrificial metal plate somewhere on the truck that will apparently rust instead of the truck. The general consensus on this forum is that it was a waste of money
Oh well, it seemed like a good idea at the time and we'll see how effective it is in the long run.
As for POR-15 I've heard nothing but good things about the stuff. I actually bought a few small cans last summer and went to work on the undercarriage. Didn't paint the whole undercarriage but had to sand and touchup a lot of small rust spots. Definitely gonna check the undercarriage and touchup this summer again.