I don't know anything about the internals of the alternative JDK and seen as it's available free now I don't have any reason not to try it. We have reasonably good test coverage. A lot of the applications on the server are groovy and grails based.
I am considering this because I've switched to using it for running all my local workstation applications such as intellij and builds and it appears to have made quite a difference. I have to admit I haven't been particularly clinical about my tests though admittedly.
Perhaps it is because things like in IDEs and builds are very I/O intensive as apparently this is one area where jrockit is quite different.
There is only one development team for Oracle JDK and JRockit. Many of the developers are from the original JRocket team. I don't believe Oracle is interested in maintain two releases.
I have found builds are IO dependant and the most critical factor is the speed of your drive (not the speed of Java) The best thing you can do is buy an SSD drive. The 240 GB models tend to be faster than the smaller models. Make sure you get the sector alignment correct. ;)
A typical 7,200 HDD can do 100 IOPS whereas a fast SSD can do 80,000 IOPS. http://en.wikipedia.org/wiki/IOPS
For an SSD, create your partitions using
otherwise you can lose half your performance or more and reduce the life of your drive.