Yes... I do it all the time, and would suggest getting every bud u can to mature, and although u need to flush it before u take the tops, u can re-feed it with some P-rich teas, after the tops r harvested, and let the bottom growth mature, and then chop it all, after yet another flush... Week 15? Must be a long flowering Sativa, like I have a couple of at week 5 right now... I also have some knarley hybrids that I bred myself, but I love the big ol' sats... If u r having light penetration issues, however, for your next grow, I'd consider not only using HPS for my main flowering light(s), but also get a MH for deeper penetration, as well as more trichome production, as this is the plant's natural response to UV radiation, which MH's do emit some of, resulting in tighter internodal spacing, as well as more potent medicine... And when u use both types of HIDs in conjunction, like I do, with more MH than HPS during veg, and more HPS than MH during flowering, you will notice much happier plants, by increased vigor, growth, size, potency, and yield... And if u can add a CO2 system, even a small, "clandestine" setup, like a fermentation style rig, u will notice an even greater difference than if it were not there...