The majority of traffic in SEP is related to content downloads each day for antivirus and IPS definitions etc.
In your case you have a very fast WAN running at 100MB so I would just treat your sites like local subnets.
1 x SQL server
2 x SEPM's
SEP Group Update Providers (GUP) at each site
You need 1 or 2 GUP's present on each subnet.
All of the SEP clients will communicate with the SEPM's for logs and policies but will get content from the GUP's.
6 SEPM's with replication between all the sites is just asking for trouble in my opinion.
If you are really worried about bandwidth during working hours you might consider it, but the sites still have to replicate at some point.