You are correct. Postfix (and MTAs in general) are extremely disk IO intensive. If you're looking into improving performance of the system, I recomend you look into a faster disk subsystem. A lot of work can be done tweaking Postfix for optimal performance too (running a second instance for defered emails, tuning the # of processes and SMTP timeouts, moving spools onto their own disks (or at least separate partitions), if you're willing to risk loosing data in the event of a power outage you can also try using a ramdisk for spool, etc...)

You may be able to squeeze a little more performance out of your existing system by generating (and delivery to postfix) mails in parallel. I was running on a e450 for a while with 4 CPUs and postfix and I found that my optimal generation rate was produced by having 8 simultaneoue mail generation/injection processes. Your performance may vary, but its worth looking into. If you try this, be careful not to flood postfix's mailqueues. If they grow too large performance will suffer.