MustangGt40th

okay i just bought a transgo shift kit and it says on the website that it makes your tranny run cooler because of better use of the fluid. but ive also read in a few places that you should run a tranny cooler as well. if i dont use a tranny cooler will the shift kit cause my transmission not to last as long? and if i do need to use a cooler how much would it cost to get one installed? im considering the mishimoto off of americanmuscle.com