Torsten, 
I was just checking the output of the convert_warp tool.  As I understand it, the "--fractional 1" option should equal the original transformation, so that when applied to the original stack using the reformatx tool they should give the same final outcome.  This is not the case for me.  What am I missing?

The commands I am using are:
convert_warp --fractional 1.0 warp_original.xform warp_fractional.xform
reformatx -o out_stack --floating in_stack reference_stack warp_(original or fractional).xform 

Thanks,
Ruben