I use SS tanks for the reason of being able to use Hewes reels, because they are unbelievably good. A pleasant side effect was/is what you're describing - thermal transfer from surrounding water bath to temper processing solutions to perfection. I use very little water by simply dipping my thermometer into the water bath, and pour in small amounts of warm water as the temperature drops more than a half degree below my 70 degree processing temp. Works swimmingly well.
And, with the plastic lid my stainless steel tanks do not leak.