its not the steering that feels heavy when the fluid is cold but the pump that cavitates that's the problem..
If it doesn't do it now when it's cold then it won't do it with a cooler. It won't get any colder. Usually the restriction of the cooler won't cause cavitation since the pumps pull from the reservior which should be full.
In my rig, the pump cavitates on starts that are -10 or below. I know it is bad, but those types of starts are rare for me (less than 20 times a year maybe?) and my pumps are cheap. I do know that without the cooler I was totally cooking the fluid after a few heavy wheeling trips before... it's also overheat and cavitate while wheeling.
For me, the tradeoff is good.
If it worked before for me (as in didn't overheat and cavitate while wheeling) I don't think I would have added one. I would be ok replacing burned fluid as long as it worked while i was out.
my rig has a saginaw pump and a stock 60 box with a tranny cooler that is probably 10x12 on it.