I installed a renntech radiator on one of my customers cars last year, a 93 modified 500e.
The radiator was a nice high quality unit,but a snugfit. It is thicker than the stock unit and must be carefully installed as there is not much space between the fan and radiator anymore. This car saw alot of track time and also competes in the "one-lap" event. There weren't any problems with the stock cooling system,this was installed to get ready for future hp mods and my customer is doing the car in "stages".
I also had the E60 that Hartmut(owner of renntech)drove in the "one-lap event" this year at my shop and I had a chance to look it over closely. The trans cooler is really "cool" having a temp sensor and electric fan mounted on it on the pass side of the front bumper and the oil cooler is on the drivers side. Unless you are making alot of hp or racing your car,these coolers are not needed for a stock daily driver.