For what it's worth here's my situation. I just bought the Jobu replacement foot for the 300 II - LF-C504M2 for about $100 and it's good. It does have the lens sitting a little high so it balances but not when the lens is tipped far down or far up. The jobu Jr. 3 is light and more than adequate for the 300 and extenders and it is very smooth in operation and locks up nicely. I'm very pleased.
This shot is my treepod with the 300 X1.4 on the Jobu Jr 3 gimbal.
Thanks for that Jack.