Very curious. Now I'm wondering how many O2 sensors my truck has. the newer ones have 3 (one for each bank, and a 3rd because...) so I'd hope I wouldn't have that many. I hope I only have one downstream of the y-pipe. Or maybe it only has one, on one downpipe. A crap-shoot I guess. But if it helped with at least 1mpg in the winter, $30 would be saved. Barely. hmm. 17mpg vs 18 mpg over 4000 miles at the prices here is only $35 saved, in 6 months. Becomes even less when I step my mpg to 18-19. Guess there's just no winning with a high clearance truck.
But awesome for you! I think in winter months (despite you not really having "winter") you should see a gain. The higher that temp differential is, the more effective this mod can be. Although it may take a few years to pay back the cost of the sensor at your mileage if the gains are minimal.