IMO, a better interface would be:

```xml
 <tag name="sonata.admin" group="Content" label="Books" manager_type="orm"
         fastGrid="true" searcher="elastica" search_index="acme.book"/>
```

For the "basic" transformer, and:

```xml
 <tag name="sonata.admin" group="Content" label="Books" manager_type="orm"
         fastGrid="true" transformer="my.custom.transformer.service" searcher="elastica" search_index="acme.book"/>
```

For a customized fastGrid.