HI,
I can't help you directly but I have a Remus exhaust system on my F56 Cooper. It uses the same front muffler (or straight pipe for deleting that) than the S or JCW model (for installing it on a Cooper you need a "A8" spacer to fit it on the smaller diameter at the downpipe/cat).
For installing this exhaust (should be the same pipes for all versions) on a F55 you always need a spacer "E8 distance tube" to balance the longer length of the car and the exhaust piping. Most probably this will be the case as well for the system you have in front of you now: F56 + E8-spacer = F55. Means that it should fit on your F56 without the spacer.
But why don't you give a call to the Remus technical hotline (Tel: +43 3142 6900 - 0)? They have super nice people and you may just ask your question and get an useful answer that way.
Cheers
Axel
BTW: don't compare the dimensions with the OEM system as the whole size, the length of tubes as well as the position of the joints and mufflers is totally different!