I have mounted the big (5.3gal) Acerbis tank. Install was very straight forward. The overall width is actually narrower than the shrouds on the stock tank. I did wash the tank out thoroughly and dry it to remove plastic shavings from the manufacturing process. I do notice a bit of "fuel slosh" when it is full, but most rides I just run it with between 3 and 4 gallons. I see very little downside to this tank.
One thing I have noticed is that since the large lobe on the left side of the bike is larger than stock, and that is where the pump resides, that the low fuel light comes on with quite a bit of fuel left. No big deal, since it is very easy to visually see the fuel level on this tank. The plastic is more translucent than the stock tank.
Most of my riding is on double track hard pack, forest road, blm roads with occasional single track. I might pee my riding pants if I came across a descent like the one Ned showed at the beginning of the post.