I would would add creatine to your line up. When I was in HS creatine took off 2-3 seconds of my 400 M (in track not swimming). I would think the results would be even more signifigant with swimming since the added musce hydration would make you more boyant and the better creatines out there. You could try supplementing 2 grams of L-Carnitine a day that has shown to increase peak treadmill spead in a group of marathon runners 5.7% over 6 weeks. Also glycerol could also help. I would hold of the xenadrine till competetion time this way it hit's you you harder.