The BDGT does not adjust AFR's which the headers will throw AFR's off. I have been there and done that personally and ended up with Uprev. Non flex trucks go lean and flex trucks go rich either is not good for the life of the engine. Headers and BDGT with bpipes and a cai on my 05 flex made 311HP and the rear tires and Uprev woke it up from there. Video of that dyno is on my website www.cajunbpipe.com. I think someone reported 20hp with headers alone somewhere. I recommend Uprev to run headers safely.
Actually headers are less than 500. Uprev is 550. Those two combined are still cheaper than OEM nissan replacements, and also cheaper than trying to run a BDGT long term with wrong AFR trims/targets. Your fuel mileage will suffer and the truck will never be 100%. And its more than 20hp at the crank.
Sell you're BDGT and make some money back, now you're in it all for less than a grand. Its a worthy and logical investment in the long run. Headers, Uprev and trutrac with axle vent mod, you will have a solid and long lasting powertrain. The rest is preventative maintenance and wear items.