Java 8 is still in beta or release-candidate (RC) status, so not official yet, but beginning with mid of March it shall be going final.
I don't know how long it will take the Raspbian developers to include the new Java 8 into the Pi's apt-get packages then.
(The current Raspbian's Java 7 is version 1.7.0_40, when the one we can download from Oracle's Java site is 1.7.0_45 -- so not that bad, really, and I'm happy we have the fast Oracle Java in the apt-get packages at all.)
So far I didn't remove Raspbian's official Oracle Java 7 package, but just downloaded the tar.gz archive for Java 8 RC (b129 from February 2014) from here: https://jdk8.java.net/download.html
Then I unpacked it and (sudo) moved it to /opt/jdk1.8.0/
Since usually you have some .sh script to start your (bigger) Java applications anyway, I think this parallel "installation" or "untar-isation" of Java 8 to /opt works fine next to the apt-get's Java 7. You can happily use both JVMs in parallel, meaning the command "java" without path will take the Raspbian's standard Java 7, whilst "/opt/.../java" uses the new Java 8 beta then, as in your latest code quotation.
Is your Java 8 RC also b129? The Java 8 beta which the Elinux site mentioned seemed to be a year older.
Regarding speed improvements: Haven't done any number benchmarks yet. Usually new Java main versions bring speed improvements indeed. Would love to read about some. However JavaFX, which is new and built-in to Java 8, delivers hardware acclerated 2D and 3D to the masses without external native bindings, also on the Pi. You probably want to check it out for own projects.