Yuri Levin emailed us pointing out that we didn't use an updated version (2009 vs 2004) of substrate thermorefractive noise that includes contributions from the standing wave in the beamsplitter. Unfortunately, this has pretty major implications for our noise budget. Attached is the updated noise budget and a 2 page summary of my calculations today and some future outlook.
Another solution not mentioned in my document: cross-correlating two interferometers. Of course this was already the plan, but maybe we should push harder on this.