HA mode causing mail issues?

I'm having a peculiar issue but i'm not sure where to post - since this forum covers HA topics, guess its appropriate:

I have a pair of 100D running on firmware v5.2.8,build727 (GA)

- connection is on a 500Mbps symmetrical fiber - each subnet is rate limited to 100Mbps

- environment has 2 subnets reachable to one another

- estimated around 4-500 devices

AV/IPS/DLP - On

Webfilter - off - when on - extremely slow performance - understandable as the 100D may be reaching its limits

Scenario:

Mode - Active-Active

- Users are complaining access to the internet is slow

- Users are on POP3/IMAP mail but are getting odd errors on their Outlook client, initially, this setup has run for close to 2 weeks with no changes to the infrastructure until today.

My goal was to load balance the processing, but I understand for this scenario, it might give some problems - could this be one of them?

Mode - Active-Passive

- everything's back to normal

- CPU usage high due to appliance reaching its limits

- Email client does not exhibit any errors

I do have a 200D which i'm planning to put as passive solely to run AV / Web filtering - would this work?