Hi Jay,
using a Template MM to quickly generate dashboard and alerts is a best practice in APM 9.x and previous versions approach. In this way you can deploy these objects reducing the time consuming that is typical of MM implementation.
I’m envolved in a huge APM project (3700 Java Agents), started 10 years ago, so now we worked a lot with MM templates (and we are still working now with these objects, but we are moving to the new Team center approach).
We built one template for each kind of AS and we deployed a huge number of MM, using copy and paste technique and modifying the Agent expressions defined in the “root” of MM.
Under the MOM load point of view my suggestion is that you have to limit the number of calculators and you have to use, in the MG, narrow metric expressions.
In my experience I have to say that moving toward the APM new versions (we started with 8.x and now we are on 10.5.2) the behavior of APM infrastructure regard the management of a large number of MM objects has improved (you can see this improvement when you save a new object: with the older version we had few seconds with no data on dashboard, now the MOM, starting with 10.5.x, responds very quickly, without HW changes on the APM infrastructure).