I went to a lower 1st gear too ...
I too went with a smaller front sprocket for the same reasons. I'm very happy with the changes for the dirt. Slow technical stuff is much more manageable and getting restarted on steep hills doesn't require you to slip the clutch much.
6th gear, in my opinion, was too low even before the gearing change.
I've put about 2200 miles on it since I put the sprocket on, but haven't paid attention to the mileage.
I'll be doing some riding next week and see what I'm getting.
2005 KTM 400exc