ive done about 305 mhz on my gskill TCCD at 2.5-3-3-6 benchstable.
But alot of these settings also depend on your cpu's mem controller.
If you cant run DDR600 at 2.5-4-4-7 then you can actually RMA your ram since thats its specs already. Cas of 3.0 will hurt ur bandwidth too much. Its absolutely not worth it.