I've found the most growth from XM 10K bulbs, but far better color with other 12-14K bulbs.
You will have to get new ballasts for the 250W bulbs, and make sure that the larger bulbs fit in your fixture.
6500K is REALLY low for a salt tank, they don't have much in the blue spectrum that coral likes, and lots in the yellow/green that algae likes. So you will be helping your algae more than coral, and algae really doesn't need help
Use your PC lights as all actinic for color, they have such low light output that putting "grow" lights on there won't be helping nearly as much as the halides.