We will be happy to open some outbound support incidents and take a look at some servers to evaluate. We have all historical stat information on CPU, Memory, Disk I/O and can verify utilization before and after the upgrade.
Keep in mind right after the upgrade SM 16.x does update accounts to new data formats etc. It does this in the backgrounds and prioritizes accounts based on usage etc. Depending on how many users you have is how long this takes.
Also, we can evaluate the speed of the web interface on your installations etc.
For shits and giggles I have asked for a couple new test environments setup for 15.x and 16.x to compared speed under significant load and large mailboxes etc. We have done this numerous times and will evaluate again.
If you are interested in us taking a look at your install, please let us know.