Instancing can save on memory but may impact performance such as time to first pixel (how quickly a render begins sending pixels) since Katana may process many nodes. Options where you can instance a hierarchy will be more efficient for time to first pixel but you may lose control of some attributes you may wish to override, this is the trade off.

In the PrmanGlobalStatements you can specify prmanGlobalStatments.plugin.flattenInstanceSources Or you can use PrmanObjectStatements with prmanStatements.traversal.flattenInstanceSource These options tell Katana to keep instances as groups rather than flatten them, this speeds time to first pixel as the flattening doesn't happen for a possible deluge of instances.

You can read more about nesting versus flattening on the main Instancing page.