You're right about them seeking slack water, but not for the reasons you mentioned.
Slow current isn't any warmer then fast current, not the fish cares one way or another: they're ectotherms, or "cold-blooded." In other words, a smallie is whatever temperature the water is, it has no way of self-regulating its body tempertaure.
To that end, cold-water isn't uncomfortable to a smallie. But what it does do, is slow the fish down. The temperature-dependant, metabolic chemical reactions that occur within a fish are slowed down when the temperature decreases. Digestion slows, reflexes slow, muscle movement becomes less effecient/effective.
This is when river smallies move into slow, to no-current areas. Keeping position in fast current (or any current for that matter) becomes a real chore when you are sluggish. The energy needed to keep position stays the same (or may even increase as the density of water increses with decresing temp), but the ability to create that enegy within the muscles decreases as metabolism drops. Add to the fact that the amount of forage needed to fuel the fish becomes more and more scarce as the fall wears on, and you have a recipe for over-winter migration.
