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If your products are ranking well for their target keywords, I would try and avoid changing their URLs (especially to a different domain). Keeping on your own domain also helps improve your product rankings when you grow your main website. Not sure how the masking will be set up, but in some cases there is no "internal links" since they are all masked by the root masking domain. This would definitely prevent you from building more links (and further improving your rankings).

Hi RAIN Group, It sounds like you are referring to the data on the Keyword Difficulty tool. That tool is actually not connected to Google Analystics, but the search volume used to be provided through the Google AdWords API. Unfortunately, Google has removed our access to their AdWords API so we are no longer able to provide that data. The good news is, we don't actually use the Google AdWords data in our calculation of keyword difficulty; it's just supplemental data, so those reports are still 100% accurate - they're just missing the information we used to pull from the API, which you can get at https://adwords.google.com/o/KeywordTool in the meantime. We are working on finding another source to partner with to provide search volume in the future, but I'm afraid we don't have an ETA for when that data will be available again directly through the tool. Thank you for your patience while we work on this! -Chiaryn