Well, for what you want you aren't going to be cutting weight, you'd be going on a diet and stepping up cardio and all that stuff. You have to ask yourself if 15 lbs. makes that much of a difference to you for the sprint. I'm not sure how much the 15 lbs. would affect your sprint time but you have to weigh the costs/benefits (I'm an economics major, we do this all the time and my professors always tell us to use this in real life when making decisions too). For some it can be very hard to lose weight and can put people in a stressed mood, which isn't healthy mentally. Do you think the seconds difference in sprint time will make you feel justified in losing the weight (when you don't really need to otherwise it seems)?
Honestly, this is what I'd suggest: do some plyometrics (for explosiveness) and continue weight training and cardio (including the Beep Test/Shuttle Run) while watching your diet. If you lose weight, great, if not, okay, but you'd be training already for what you want and your body will respond.