Yes, that's pretty acceptable, and generally something to be expected with a mail filter.
Our setup is a bit different. We have a separate server for SpamAssassin, while our POP server runs ClamAV to scan for viruses. The POP server is generally running under a server load of 2, but occasionally spikes up to 10 or more. Our SpamAssassin server on the other hand, used to run around 2 until we also installed the Openprotect.com filters, when it doubled the CPU usage and is now running under about 5 with spikes above 15. This is still acceptable because we don't have any delays in mail that result in a growing mail queue (we use qmail for incoming SMTP), and there's still room to spare CPU usage/memory wise.
Coincidentally, I highly recommend Munin for monitoring your servers. It does a great job of visually demonstrating historical data and showing you what resources you have to spare. Monitoring in real time with Top(1) doesn't help you much. :)
Oh, and by the way, deploying on the Friday before the long weekend is a great way to work through the whole weekend. Especially for critical systems like a mail server.