We run one of the largest SmarterMail installations that they support and the SmarterMail build we are running right now performs extremely well. CPU usage is basically nothing compared to some of our other systems, such as web servers, SQL Server etc. RAM usage We sit around the 15-20GB usage area, but we certainly do not mind increased RAM usage if it translates into better performance. Disk IO across our RAID10 array is also extremely low, it used to be an issue, but lately SmarterTools has done a really good job of optimizing SM to not be so hungry for resources.
We have 2 anti-spam gateways that run rspamd, they feed our primary SM Server incoming mail, we have a linux system that runs ZoneMTA that handles all the outgoing mail. One day, we hope to dump the ZoneMTA system, just waiting for the day SmarterMail supports and can do its own outgoing IP rotation.
Our system size is well over yours and we have no problems at all with SM performance.
The vast majority of our users are Outlook and Tbird based, running IMAP and POP3 + SMTP. We do have a good chunk that uses the web interface exclusively, and decent size number of users who use Exchange Web Services (eM Client and Apple Mail). Getting users to pay for access to their email for something like ActiveSync is a hard sale.