You're right. Google lends preference to relevant regional results when the search is performed on the regional server (Google.ca versus Google.com).
If the links are exactly the same in your .xml and .php files, Google may see it as duplicate content. Due to an oversight, I've just encountered this myself and I'm hurtin' pretty badly for it.
If you're hoping to expand into the US, it's time to start building links across North American web directories and expand your advertising to include US servers.
Google's new algorythym now appears to evaluate the relationships between your site and the sites that link to you. If they're in no way related or the sites that link to you are link farms, Google ignores the links and your rankings drop. I've noticed that over a hundred inbound links from unrelated directories and sites have gone missing in the last few weeks.
Try to accumulate links from related sites and your relevancy should start to climb.
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