A con for the C63 is the head bolts blowing up; at least on older models. MB changed the part number for the bolts.
911 is way too small in back. You'll never get a rear facing seat back there. Even in the E9x, a rear facing seat will be TIGHT up front.
Let me get this straight... You are swapping out parts designed by some of the top engineers in the world because some guys sponsored by a company told you it's "better??" But when you ask the same guy about tracking, "oh no, I have a kid now" or "I just detailed my car." or "i just got new tires."