The reason why most have high capacity PSU's is because of one of two things:
1. They have power hungry components (not related to the watercooling gear). This means top tier GPU's and CPU's that are typically overclocked (sometimes heavily at that).
2. They buy more power than they need for various reasons. Most of the time, people do not know just how much power they need, exactly.
Really, your PSU should have no problem support a watercooling setup like the XSPC Raystorm kit. All you are adding is just the power needed to run the pump and any extra fans.