as far as a tranny cooler, I would definitely reccomend it. I tow alot; work trailers, jet skis, boat and it is just that extra peice of mind.
As far as voiding the warranty, the dealership has to prove that the cooler ruined the tranny. Since it is the purpose of the cooler to prolong the tranny it is almost impossible.
I got a high performance heavy duty cooler from Uhaul onstalled for about $120.