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We've done this before and usually, we get dips for some keywords, most will stay the same. We prepare ourselves mentally each time we have to do this LOL. As long as the redirects/content are in-place and you provide a better user experience without affecting load speed, then we are confident enough to just go on and do it. The rewards for the "risk" is always greater.

Andrew, I wouldn't put crawlable versions of reviews found on other sites on my product pages for the same reason you are concerned. You could put them in an iFrame, which should take care of the issue. However, in order to get the star ratings in the SERPs the Schema markup for aggregate ratings should be outside of that iframe. An aggregate rating doesn't include the review copy since it is including all of the reviews (e.g. 4 out of 5 stars based on 320 reviews). Meanwhile, I would be working on getting your own unique, real reviews up on your site so you benefit from the user-generated content that nobody else has.