It's odd Stern haven't stated 600LEs on their website. My little bet here is that they will do 1000 LEs. If Stern haven't publicly committed to 600, they seem to deliberately leaving that door open to do more than 600.
...I'd love to see 1000 LEs. It will show that the pinball market is really is growing. It will be great for pinball to see Pro/Premium/LE out sell ACDC.