Is that remotely true? It's been a long time since I benchmarked it but groovy performance (outside of primitives) was dreadful last time I looked. I'd be pleasantly shocked if it could overcome the dynamic language costs.
Not sure when you last looked, but it gets a performance boost from invokedynamic on JDK 1.7 [1] and another from @CompileStatic [2]. Groovy 2.0+ is much better than previous versions.
Related, Grails 2.4 adds support for using @CompileStatic in controllers and services, whereas previously you had to keep that in separate classes in src/groovy.