It's very good to hear that you have managed to hire such an intellegent and talented developer
In most cases we don't consider Groovy as an alternative to Java. We look at it as an ajunct to Java.
Many Groovy users develop the whole of their application in Groovy. They find the development time considerably shorter than in Java and the source code is lots more compact and lots less "noisy' so it is easier to understand what the code is doing.
Often performance will be satisfactory at that point. Especially if the application is making use of substantial Java libraries so that most of the processing time is spent in Java generated bytecodes.
If there is a performance issue then it's pretty easy to profile the app and find the hotspots. It is then really easy to move the this code into Java. For example if you have a method on a Groovy object which is taking a log time to execute just subclass the Groovy class with a Java class and override the method with a Java implementation.
Just over a year ago I was consulting with an early Groovy adopter who has a 40,000 line Groovy application which is key to a part of their business. They had a performance problem and were not able to meet their response targets. I was able to use Groovy's dynamic capabilities to instrument the application without changing any of their code. We found that the problem was that BigDecimal division was taking a very long time (it's a financial application). Recoding two methods in Java (less than 50 lines of code) fixed the problem. They have gone on to extend the application and are very active members of the Groovy community.
We have had on Groovy user who implemented a signal processing application in Groovy. The performance was not acceptable, of course. However he claimed that his development time was a fraction of what it would have been in Java and developing it in Groovy and then re-coding in Java was far quicker than developing in Java.
Groovy is not a silver bullet, of course. However we are finding that programmers really like the dynamic nature of the language and love the ability to dip in and out of Java. Of couse, the availability of the vast set of Java libraries is a real benefit too.