Theoretical and actual delta T will be considerably different.
Cathar is very correct (as always), power draw doesn't equal the amount of heat that you need to dissipate. I've found that it's easier if you think of a TEC as a little heat pump. It makes heat...if you can get rid of that heat, THEN you get cooling. When using a TEC for cooling purposes, it's very important to remember that it's going to pump out a massive quantity of heat for a little bit of cold. They're really rather inefficient. However, they are a lot of fun to play with.
However, -60 to -70 temps with a TEC...probably not going to happen with watercooling. When you stick a TEC on top of a CPU or GPU, you not only have to dissipate the heat from the CPU/GPU, but you also have to dissipate the heat from the TEC. I don't know of any waterblocks that can keep up using "standard" watercooling (when I say "keep up" I mean below ambient temps on the CPU). You're going to need to get into the realm of chilled water to get sub-ambient temps.
I usually get between 2-4C below ambient by using an air conditioner to blow into my radiators, sometimes it's right AT ambient. It's really difficult to get rid of a lot of heat.
You've apparently done your homework though, so you should be good to go. One or two TECs can usually be overcome by watercooling as long as they aren't massively overpowered.
Good Luck with your setup.
-baron_iv