Most Rialta owners have installed the trans cooler as the transmission is the weakes link in the drive train. The orginal pan cake cooler is cooled by the radiator water and keeps the temperature of the trans fluid at 190*F or higher. If you are climing a hill your water temp will get to about 220*F which will reduce your transmission fluid life. VW stated that trans fluid was lifetime but changed after replacing lots of tranmissions. If your tranmission fluid gets too hot you go into limp mode (3 gear only) until the fluid cools down. Trans fluid life is reduced by high temps and will cause shifting problems if not changed. VW recommends changing fluid every 40K miles. The external cooler replaces the pan cake cooler which keeps your trans fluid from 150*F to 180*F temp for long life. I did my own replacement or you can have a shop do the replacement for $$$. Here is the trans cooler that is used for the R.
Also most have added the tranmission thermo bypass to allow the trans fluid to warm up faster and for cold weather. I feel that this is the best modification you can do to protect the transmission, as a rebuilt trans is arounbd $5K.