I am working on a website where we are encouraged to copy the text from the manufacturers website. How much will using 100% copied text affect our google position for product keywords compared to if we either edited the text or completely re-wrote the text?
I would write your own original copy, its worth the effort... Google doesn't give credit to duplicate copy and you won't do as well in the search engine results.
Does the OEM allow it's resellers to do that? Violating the OEM's copyright like that might cause your client to lose their reseller status or face a lawsuit for copyright infringement. If it were my client, I would explain the waste of time SEO-wise it would be to copy and firmly state as a writer I only write original content and do not simply copy other people's work. Even re-written content would be protected as a derivative work. If they still insisted, I would fire them as a client.
copied text worth nothig for search engine traffic, you will not improve your rankings. But nobody prevents to test other method like ppc.
From someone who has direct experience managing the marketing of ecommerce stores who face this very situation, the copied text is not that big a deal. Ideally the product description should only be part of the page content. Adding other details to that page makes it unique from competitors' pages. In some industries you are forced to use some duplicate content. If duplicate content were that big a deal, every website that quoted religious scripture would have lousy rankings.