Wow, GM really pulled a boner then, all of these then should have come with smaller pulleys. There's a reason the pulleys are sized they way they are, it wasn't GM engineers going "well that seems OK" they're sized to provide a specific water flow, water flow is paramount in a cooling system regardless of thermostat, once it's open, it's done it's job. If your temperature is running 200° or so the thermostat is fully open, reducing water flow will reduce cooling efficiency.
Now what happens, the fans run more to regulate cooling instead of the system working as designed. Where does the power for the fans come from? Let me think.... oh that's right, the alternator which is now also under-driven.
Do what you want if you think you're going to gain a lot, personally I'll keep the stock pulleys and find more power elsewhere. I'm really skeptical of the claimed power increases. Not only that but the pulley is also the damper, why stick some POS pulley in place of your engineered by GM pulley? To gain what a few HP? Not worth it to me at all.