Correct me if I’m wrong but, for mirrors especially, can’t you overlay texture from RTT on top of texture from reflection probes? Instead of having to render the whole scene for the reflection you only render objects that aren’t a part of prerendered reflection and then overlay it to save a heck-of-a-lot of render time. I hope I’m making sense.
I think I understand what you’re proposing – the issue we’d have is that the pre-rendered reflection probes are only roughly reprojected to look like they’re in the right place. It’s not just that things are missing from them, they’re very inaccurate, so adding in other objects would look very weird. Even without accounting for that, there would also be a problem that if, for instance, there was a table in the pre-rendered reflection and we wanted to add an object behind that table, we wouldn’t have the depth information to composite them correctly.