Hello,
I think you miss one critical decision criteria here, which is the accuracy of the spam filtering and the amount of possible false-positives. If you look to independent analysts report such as Gartner Magic Quadrant, Radicati Market Quadrant or Forrester Wave, Symantec Messaging Security products are always up in the leading quadrant, whilst Trend Micro isn't. So why would you like to switch off an antispam engine, that gives you much higher accuracy and lower amount of false positives just to overcome overhead and hardware resources?
You might want to consider the migration to Symantec Brightmail Gateway 8.0, which gives you inbound and outbound messaging security for email and IM, with effective and accurate antispam and antivirus protection, advanced content filtering, and data loss prevention technology. So spam mails won't even hit your Exchange environment and can be quarantined separately for further inspections by the user or the admin if you need to (in fact, many customers switch this quarantine off after a while because they realise the accuracy of the antispam filtering and they realise, that the mail users are not looking into the quarantine anymore to check for possible missed items).
So on the Exchange Server itself, you can run antivirus/antispyware engine, i.e. Symantec Mail Security for Exchange. Brightmail Gateway 8.0 and Mail Security for Exchange is part of Symantec Multi-tier Protection bundle and it is available at a very attractive crossgrade price from your current Brightmail 6.2 version.
Running the antispam engine on the Exchange Server itself is not a recommended solution, because it doesn't keep the spam and malicious code overhead away from the Exchange environment. Doing it on the gateway level is a better approach.