If you have an even canopy of buds up top, the canopy will get very sufficient light and will produce strong top colas. Any side branching that makes its way to the top will have equally as strong light, as long as it's not being shaded by other buds. However, the lower buds won't get as large and may mature a bit slower because they're not receiving as much direct light.

A 250w metal halide is fine to add to the room, as long as you can keep it cool. Try to keep the temperature of the room under 85F. In my opinion, more light is always better in flowering... regardless of whether it's HPS, MH, or fluorescents. The spectrum of an HPS is ideal for the flowering cycle, but they'll flower under different spectrums as well. Again, as long as the room doesn't heat up too much, the plants love the added light.

Next time you veg, you might want to think about possibly LST'ing the plants to produce a dense canopy of buds, or making a SCRoG (Screen of green) using fewer plants. As you transition from grow to grow, you'll start finding new ways to make the absolute best of the lights you have available.