The scanguage is worthwhile... that difference in mileage could be due to nut behind the wheel adjustments, like any of the mechanical issues TedV mentioned.
Even if you (plan to) drive a lot, it'll take a few months to a year or so to pay off the difference in cost between 43 and 50mpg. So, it's a longer term payoff.
I know in my case, my SG-II helped me fix bad habits that were eating more fuel, when the price of fuel was climing and climbing in 2008. That, and I discovered that my TDI was overstating it's speed by about 10%.
The downside, which you'll find out reading here on Ecomodder, or on TDIclub, is calibration of the SG-II isn't exact. If you vary your mix of highway vs. city driving from tank to tank, it's going to wobble above and below, as you try to correct for your current fill-up results.
That caveat aside, I found that the SG-II both teaches you the better ways to drive more efficiently... and you'll discover that some of the more aggressive ways to drive (at least until reaching a cruise speed), can be MORE efficient than a slower, more conservative approach, with a TDI.