BUT if your server is US based you perform better across ALL regions
I do not know the answer however a year ago I moved a .com from a UK server to a US server, after about 8 months I moved it back to the UK since it had no effect whatsoever. This specific site used to reign supreme across all G.tlds but 30 months ago G decided to send the index page into obscurity even though it is the de facto trademarked widget industry site that has been in existence for years.
Sometimes this site re-appears at #1 in Google.co.in, it is an Indian product exported globally, however most of its traffic is my widget trade requiring necessary information. All other search engines list it as #1.
it raises some concerns about how google is sculpting traffic
I feel you're being kind calling it sculpting, I prefer deliberate manipulation in favour of US companies. So many SEOs outside of the USA are seeing this and it will come back to haunt G and bite them extremely hard if they don't rectify their ways.
At one time I considered my loss of US traffic was the "localisation" of their US SERPs however if this were to be true just WHY would widget US importing companies rank so well in "foreign" Google.tlds ... Quite simply localisation cannot be true, outside of the US, since these US companies ought not to be appearing IF localisation were a critical factor.
One thing I will say in G's favour is that whilst searching for my keyword1keyword2keyword3 term it's a mess however when I search for kw1kw2kw3 +my county area or city, the results are absolutely spot on, all good local suppliers and not a US one in sight and no, I am not ranked but my sister's business which is the area's pre-eminent retail supplier, is.
So, they can get it right but how many of Joe Public use 4 keywords in a search?