the 2500k turbos up to 3.7. So if you can make it stable at a consistent 3.5 or 3.6, that's probably as high as I would go with a stock cooler. Maybe not even that far, really. You're going to be generating a lot more heat than the stock coolers are made to dissipate.
Whatever you do, make sure you do a lot of stress testing before you call it 'stable enough'