You don't want to use a resevoir, you want to use another waterblock. The heat transfer will be higher if you use a waterblock because the water will be moving which increases the convesction heat transfer coefficent. The flowing water will also increase the delta T between the water and the pelt.
This is exactly the same reason you want a waterblock with high surface area and flow on your cpu. Imagine if your waterblock was just a big copper box filled with water, wouldn't work to well would it.