We hadn't heard of specific third-party lens focus issues while using the 5Ds / 5Ds R (until now), but it doesn't surprise me. I'm a huge fan of Sigma's Art series (and own 3 of them), which is why I'm glad I can update their firmware – if needed – at home via the USB Dock.

Let me know how well the Tamron works in Live View after you get it back.