Hi Thomas,
PLA’s are pretty good in certain conditions, but they’re not as fast as they could be - I did a number of benchmarks and they were never the fastest or best:
I get what you’re wanting though - a scene which you can edit as assemblies, but it renders as all ISMs or HISMs for speed. That’s why I created rdBPtools (on the marketplace) - it’s made to create nested assemblies which can all be assimilated into just one actor and global HISMs - when using the C++ rdInst instance handler, it’s the fastest way by a lot!