Heres a front and back view of the 416 400gn federal fusion in between a .375 270gn TSX and .375 300gn Patition.
The last photo is a side view of the fusion next to an unfired 370gn North Fork. the heals of the both bullets are lined up so you can see how far back the fusion pealed back
From your tests, it would appear that the Fusion would be a good bullet for cats. I had an A-Frame in .416 Rigby zip right through a lion that resulted in a, shall we say, exciting follow-up. The necropsy revealed the A-Frame, an excellent cape buff bullet in the same caliber, simply failed to expand. The following year, I used a Partition on another cat hunt and it worked perfectly.
I wouldn't hesitate to use a Fusion on cats (for the first shot, but wouldn't use on on buff. Thanks for your info and teats.