I read somewhere very recently that if you redo the medicals unprompted by the visa office , that it could cause a "serious" delay. Reason being is that each completed medical is treated as if you are a brand new applicant since the certificates come with a unique identifier and likely cause confusion for CIC (regardless if you indicate your UCI number). However if you redo the medicals when the visa officer requests for it, they likely have their system set up so that it would easily match up your existing file with the redo at the requested time.
This makes sense since the website clearly states that redo's are not encouraged until instructed to do so by CIC. If I can find this info, I'll certainly post for you.