The bulb isn't a metal halide conversion right? Still an hps? Your plants are probably growing more because of the sudden increase in available energy. You did increase it by a nice 67%. Other than that, the plants (mainly the shorter ones) are also attempting to reach the 'optimal distance' from the light source. I keep my hps about 12-14'' from the top colas. They'll tell you if the light is too close. Also, try putting something underneath the shorter ones, to bring them up to the same height as the others. That way you have a nice even distribution of light/energy over the entire canopy. This will also ensure that they're not all growing at different rates due to the distance from the light. Holla holla!