I have been wanting to do the same to my 2011 w/ competition package. GTS it? I applaud the huge effort to make this a REAL thing. All the details are incredible that you are going to great lengths to match. But, The roll cage your installing looks nothing like the roll cage in the GTS.
Can you actually acquire the OEM 4.4L, if not, maybe the Dinan 4.6L? What the heck slap in 4.4L twin turbo from the 2013 M5.
Anyhow, keep up the GREAT WORK.