How to clean install Yosemite and new Photos.app

I'm using Yosemite and the new Photos.app, together with icloud Photo Libarry. I'm pretty satisfied with this combination, but I'm having trouble with one specific feature: synchronisation of face meta info. First, let me provide some information about the computers I use.

I have two computers, an iMac and a Macbook pro. The iMac is my "primary" computer. I use it for my iTunes library and it's my main "hub" in the living room. I also have all my photos on it. They used to be in iPhoto, but now they are in the new photos.app (and synced to iCloud). The Macbook (the 2nd computer) is also connected to my iCloud account and is the photos app is configured with the "Download originals to this Mac"-setting. When I import a photo on one of both machines, it gets synced through iCloud to all my devices (both iOS and OSX). So far so good. Now the problem is like this: only on my iMac the photos are showing the faces information (tagged / faces). On all other computers, the face-tag is filled with "unnamed". It seems that this information is not synchronised through iCloud.

The concrete question I have now: how do I perform a clean installation on my iMac (my primary machine) without loosing this meta information? I think I can do two things:

1. On the freshly installed iMac, simply enable iCloud photo library and let it download everything from the Apple servers. Basically this is the same as I did on my Macbook and the result is described above: it downloads all the photos, but it does not show the correct face tags on the photos.
2. Before the fresh install, create a backup of the "~/Pictures/Photos Library.photoslibrary"-directory. After the installation, simply copy this library before opening the photos.app and then activate iCloud library. But I'm scared that the photos app uploads all the photos, resulting in doubles.

How do you guys do this? Do you perform a fresh install after enabling iCloud Library?