FWIW, my gas usage this winter season so far is much less than last year, and the only thing that changed is the average temperature...this winter has been noticeably warmer than last winter, so you need to compare the average temperature to rule that out for your water heating portion of the bill.
I actually track my gas and electric use by month. I adjust my energy consumption with heating and cooling degree days to give a more accurate assessment of my overall home efficiency. After rechecking my numbers after I got home, you are correct in that we had a bit warmer weather. I'm seeing a bit improvement in my overall home efficiency rating. However, it is too early to assess the trend as, even with adjusting for total degree days, I get some wobble even on a rolling 12-month calculation.
At the end of the day, I don't and didn't expect much gas savings from switching to a much more efficient water heater. My water heater is a fairly small gas load compared to my heating especially since I use lower flow shower heads and efficient appliances. However, I am surprised it isn't more readily apparent in the data. My hypothesis is that, like reports from those who have tankless units, longer showers are erasing some of the savings.