I see your point although you can also colorcode the query-windows in SSMS. Register the instances in registered servers and apply a different color for each instance under the "connection properties" tab. If you connect through these registrations the query-windows will have a colored status bar.

If you want to look at resource-usage of each separate SSMS I don't think that can easily be done. Perhaps the "process explorer" from Sysinternals can give you a bit more insight.