Inside of the FileCacheStoreConfig class, the setter (now being deprecated in 5) for streamBufferSize has a typo when checking for immutability. This prevents the value from actually being overridden from default. While this is being deprecated, it's the only option available for those of us stuck in Infinispan 4.x for the time being.

public void setStreamBufferSize(int streamBufferSize) {
      testImmutability("steamBufferSize");
      this.streamBufferSize = streamBufferSize;
   }

Notice it is spelled STEAM buffer size instead of streamBufferSize in the testImmutability method call.