do w/kg matter in a flat sprint? I've always assumed that absolute watts would matter more in a sprint that begins from, say, 28mph.

One thing I got going for me is the ability to accelerate, but I assume that's because I'm light. I just assumed that I get beat in sprint because the other guys have more absolute watts, but then again I've never seen their power profile.

So answer the OP's question, there are many things you can do to accelerate faster. Staying low (see Cavendish) and working on your sprinting form well help you get moving fast.