does scala have any performance improvement compared to Java?
No. Scala code is exactly as fast or slow as the equivalent Java code.
if the underling JVM changes how will scala survive?
If the JVM changes, I suspect Scala will change too.
is there any limitations on scala since it is build on top of JVM ?
There are some limitations that come with the JVM, yes. For instance, there is no JVM byte code for tail calls. Tail call elimination is particularly important in FP, so we have to resort to techniques like trampolining (covered in chapter 13 of the book).