Theoretically, just static meshes is better than blueprint actors, because there is less overhead. And then instances of those meshes is better again.
Having said that, the best thing you can do is test.
Write a short script to procedurally fill your level with an example structure made from meshes. Then do it again with blueprints. You can place quite a ridiculous amount, 10,000 or so.
Do you see a performance hit? If not, then why worry?
Things don’t always work out the way you assume they will. I had a maze made out of fairly complex hedge meshes and pillars. I went to the trouble of instancing everything and performance was ok.
But, then I built the level again, using blueprints for objects, and it was MUCH BETTER.
Who knows… 