I changed to a 15 front some time ago and like you, we (wife and I and others) do a lot of dirt riding and the difference the 1 tooth made to the ability to rdie the bike easier (for a 207kg bike) in the dirt and technical stuff far outweighed any fuel consumption figures.
I found the acceleration in 6th gear is far better for us now, but our speed limit is only 100 - 110 kmh. The ability to just blast past a tintop on open road without having to change down to 5th or even 4th to get past quickly is not longer an issue.
I find that i am constantly riding in 4th and even 5th gear on 4WD tracks and using the engines low RPM torque to smooth out the ride. It is amazing how much low end this engine really has. Where as before I was riding in lower gears and the slightest throttle twist would have the TKC break traction, I can now just ease the throttle open and attack the snotty stuff without losing momentum through wheel spin. Of course the lower 1st gear is great for just crawling when required without slipping the clutch to much.
Downer for me...
One thing I have found with my bike is that the clutch will grab if slipped too much. I have duplicated this many times and dont know why it happens, but if I'm slipping the clutch to get traction on very loose steep terrain the clutch will suddenly grap and stall the engine. Not good.
Anyway put me down for a +1 on the 15 tooth front. I like it.
That doesn't mean that I dont wish BMW had made the gear ratios different though, but we use what we have.
Cheers from downunder..