Trade offs of putting an international master database in us-west-2
It's been a while since posting the question, and a comment I made above seems to be helpful despite the fact it's using a quantitative assessment, so I'll post it here as an answer.

As per comment above:

@030 I can offer my experiences since asking the question, albeit it's a subjective view as I have no quantitative data. I can offer that a year and a half later, I haven't regretted my decision putting the master into us-west-2. Our users in any region don't seem to be impacted by having the master in us-west-2, at least from a UX perspective. I suppose at some point it could be worth measuring write times -- but to date I haven't had to. There's no business impact from any latencies due to the location of the master, that we can detect at least, so we're happy here