>  in case the GRIB API were to introduce a bug in the def files

Let the GRIB API test that they aren't breaking anything.

> provides an integration test with eyeball verification

You can do eyeball verification without producing a plot every time. Testing it once (by producing a plot) and then verifying the necessary data is all that is needed. The problem with this kind of integration test is that, using your argument, I could justify a test of *any* of the iris functionality in the ```test_grib_load``` module, but that simply is not practical (or desirable). I'd ditch both the visual tests and one of the grib edition tests as they only serve to make the test suite bigger, without actually testing other parts of iris in new ways.