You will likely be blocked by https://issues.jboss.org/browse/AS7-5281. Other than that, I don't expect there to be performance implications once this is working correctly.
The current workaround is to replace the current Hibernate jars with the newer ones (e.g. 4.1.10 or whichever version you prefer).
I'm using Spring JPA's persistence.xml-less ability so I don't think this issue applies, but thanks for bringing it to my attention. - Dave