Tested the 400 with the Wimberley P-50 and found it short towards the lens mount without the 2x extender and hazardously short with the 2x and 1D4.
I guess this also applies with the Kirk LP-55, Wimberley AP-602 and 4th Generation Design CP-40 as they do not have enough plate area to clamp upon and yet maintain balance. This gets worse when you attach a 580EX II.