Your mercury vapor likely doesn't have the right spectrum of light (5000K-6500K = more blue light). You can grow quite a plant with CFLs too. Just try to find 6500K temp on the box or bulb base. CFLs create less heat, and put out pretty good lumens per watt of energy used. Plus by not getting to hot, you can get them close to the plant.

Issues with CFLs are,

First,.. You need to get a good number of them(add up the lumens). Each 27w make about 1700 lumens. You want 4000-5000 lumens per cubic foot of grow space. However with LST or SCRog you can get that easier through keeping the bulbs closer to the leaves.

Second,.. CFLs just don't produce the intensity of an HID. SO flowering with CFLs usually won't give you the bud density, or as high a yield as the HPS light will for flowering.

So in your case, use CFLs to veg and your HPS to flower!:thumbsup: Your biggest issue however(because people DO veg under HPS the plants are just tall and stretched) is you giving only 8 hours of light. The plant should be getting 16-24 hours of light per day!:thumbsup: Preferably blue light, not red. Red is for flowering.