Moving from GW8 on OES-NW to GW2012 on OES11: Does it make sense to create new post office and move users?

We have a medium size GroupWise 8 system with a single post office holding less than 600 user accounts, 128 resources and 77 distribution lists.
Due to politics we also have a non-GroupWise external domain that contains a non-GroupWise external post office with a large number of "users".

The post office is on the primary domain.
3 secondary domains hold a GWIA and two web access agents.
I've never been real clear where the external stuff exists...

One of the web access gateways resides on an OES11 server.
The other webaccess agent and the GWIA are on OES-NW servers.
All reside in NSS volumes.

Given the lack support for NSS volumes on Linux servers by most backup applications (including mine), I've pretty much determined that it makes sense to move my GW system to EXT3 volumes.

Even if I change backup software and stick with NSS, and even though all my data is stored on SAN arrays, from what I've read on the forums and from Danita's books, it's preferable to copy your GW system files to a new NSS volume, as opposed to just re-attaching the OES-NW LUN to the new OES11 server.

Point being, whatever I do will involve copying the entire system via dbcopy and at some point, will also involve shutting down the existing domain and PO for the last phase of the copying process.

So..., I'm contemplating the following scenario:
Create a new OES11 server with EXT3 volumes sufficient for holding my GroupWise system.
Create new GroupWise 8 domain on the server.
Promote the new domain to primary and upgrade this new domain to GroupWise 2012.
Once the new domain is upgraded, create new GW2012 post office on new server/primary domain and migrate the users, resources and distribution lists to the new post office.

In the past I've had an additional post office and had migrated a about 20-25 users to that post office.
It wasn't a terrible process, and fortunately, I didn't run into any problems with the migration of those users.
When I re-consolidated my system, moving them back was also problem free

On the pro side:
No downtime of entire system
Relatively simple
Any issues in the migration process would be user-specific and would not effect the entire system.

On the con side:
Process could take far longer, since you can't move a lot of users simultaneously.
In addition, we have a small group of users with anywhere from 40,000 to 80,000 email in their account, so I'd have to factor that into haw many users I migrate at one time.

The chance of running into a stalled migration, for whatever reason, is probably much higher than running into a problem with moving the domain and PO via dbcopy.

The unknown factors for me and my level of experience:
It's been a while since I've had multiple post offices, so I don't recall what changes the users might see with regard to distribution lists, web access or external email.