obviously you'll want to save the .f3d file that has the history so you can make changes. And as you've found having all that history in assemblies can weigh things down. You can copy/paste just the body into your assemblies, which will place the body in your model as a dumb DM model, with out any of the history. You can also save the body in a separate file so it can be inserted like a component, but i invariably just resort to just copy/paste. For components already in your assembly, you can convert just that component to a DM feature in many cases, which will dump the history of just that component.
short answer, A step file won't have any performance advantage over a fusion DM model that doesn't have history. And avoids possible translation errors converting the step model back to fusion model. every time a model goes through a conversion process, there is a possibility for some loss of information. in your proposed case, it gets converted twice (fusion to step, and then again step back to fusion). I would avoid that.