Thanks guys. I am pretty optimistic this will work great for me.
Since HMS told me that the E9X M3 seats have been tested to be strong enough (that's why they recommend the Rallye 4, in addition to the Rallye 3, as the back straps angle downward towards the back) and the buckles seem to be an exact fit with the E9X buckles, I am pretty sure the official E9X Quickfit harness will be pretty much the same as the R55.
Current: 2016 F80 M3 SO | AB XT1 | ZCP | etc.
Previous: 2009 E90 M3