There's a bit of a tension here, because in the end those particular parameters still live in the Nengo object itself. All that these configs can do is set the default. That's the thing that I'm not happy with about this system right now is that it's allows for several things that all kind of conflict:

```python
ens = nengo.Ensemble(...)
ens.radius = 3.0  # this one is real
config[ens].radius = 5.0  # this one doesn't do anything
config[ens].backend_specific_thing = True  # but we have to allow for things like this
```
We can just not copy the radius param for instances, but because of defaults we *do* want this to be possible
```python
config[nengo.Ensemble].radius = 2.0  # sets the default radius
```
It's possible to just blacklist those params when you're configuring instances, I just haven't done it yet. It does, though, make me scratch my head in general and wonder if this is the right way to do things.