Just taking a crack at this, I've worked with that module before and done discovery against a couple of mainframes and we didn't get MIPS. I did see that there are some extended values you can get if you have additional MainView products installed: Discovering mainframe computers - BMC Discovery 11.3 - BMC Documentation however, I was not able to find the definition of what exactly is included with "View 0YZ", (getTransaction) for example, but probably just didn't find the right document.
Also looking at the taxonomy, I didn't see an attribute for MIPS either though. Theoretically, if you knew the command to run against the Mainview agent to get the value, I suspect you could probably write a pattern to execute that and show it as a display variable in Discovery. As you mentioned though, this is only accurate for the moment that Discovery was actually accessing the system, so its value might be limited anyway.