For max sprint power, I'd first suggest working on short, 10 pedal stroke max, small ring jumps - all out. Start 39x15 or so from slow speed and work on getting on top of the gear as quickly as possible in 10 strokes. Then rest for 5 minutes and go again. Start out with 5 and work up. Give good rest afterward. For sprinting at speed - I find fast group rides excellent as you can use the draft go get up to speed and then can jump hard in the bigger gears, getting on top of them fast, and pushing lots of power.