I agree, this would be helpful.
Regarding you question though, the global thresholds are just that. They are global. There's no way to specify this on a node by node basis right now. Ways we work around some of this is by creating reports filtering out the two groups or adding two resources that are filtered to only show the two groups. Then you can sort top XX in those resources so you can see where your problems are.
A side note: though it may not be exactly what you wanted, if you configure your global thresholds for the greater number of devices, you can put a note in your subtitle for the filtered resource as to what it is.
If we can post this as a feature request, that would be great.
The thresholds you are refering to are the ones that change the color of the interface to yellow and red at the specified levels, these thresholds are only for visual effects. For threshold alerting you have the ability to configure Orion in such a way that it will notify you of 50% utilization on some systems but not until 98% on others. These are the thresholds the system is designed to use for imortant notifications, the thresholds you are using are for vistual effect only.
Also, you do have the ability to create custom reports and embed them in the Orion views. With this you can create your own custom top 10 lists based on your specific criteria. This could also likely accomplish what you are trying to do.
Hope this helps!