I am tempted. I'm just not sure what to sell to make it happen.

For those wanting a 70-300mm debate... going from 300 to 400mm doesn't seem like much, but it is. I mean, it sounds bigger yes, but not enough that it would make much of a difference, would it?. But it's 33% more range (+100/300), which translates to 77% more pixels on target (1.33 x 1.33). That's a fair chunk of extra detail, and/or newly usable shots.

I would use this when I'm focal length limited, ie: stuck outside the agility ring at the opposite side from the dog. Especially a small dog. At this distance (95-105 ft), at 300mm, framing is 7-8 feet wide, which isn't ideal for a 1 foot long Yorkie. The 400mm would drop that down to 5.25-6 feet, and the DOF would drop from 7', to 4', and the background blurring away fairly quickly.

I'd actually done most of this math already, and was considering using the 70-300L + second body w/ 400 f/5.6. I didn't think the Tamron 150-600mm IQ would be good enough, or the focusing fast enough, nor accurate enough. The new 100-400 makes things easier, as it gives me a known level of IQ, build quality, IS, and focus speed.

If they had pushed past 400 that would have been icing on the cake, but I think this lens will make a lot of people very happy.