Funny enough, I've done the exact same thing to my car, but never noticed any real mpg difference from switching the alt.
I've got a switch in the cabin so I can manually switch the alt while I'm driving, so I'd switch off while accelerating or holding speed then back on while braking.
If I switch the alt off while idling the revs alter and load reduces, but if I switch while cruising at a steady speed displayed mpg drops very slightly (according to torque pro).
With the alt off my voltage drops pretty quickly, which made me wonder if the dc-dc converter was adding to the drain on the battery and effectively negating any gain, but I've got two dash cams and glow plugs burning away too... at this point I got bored and moved onto some other project...
