You can monitor tranny temps all you want, but when it does get hot on the guage, it's too late, the tranny is fried. For an extra $200 for both an extra cooler and external filter, you are just adding to the safety and longevity of your tranny. I have the external filter on my wife's Lincoln LT as well. Neither have ever leaked. When your tranny goes south, it will cost upwards of $4,000 or $2,000 at a minimum. I also recommend a Transgo Tugger Shift kit for around $100 that replaces a lot of the factory plastic parts and firms up the shifting and also saves the internal clutches.
2008 F-250 4X4 Lariat V-10
2002 Airstream Classic 30' w/SO #2074
2007 Kubota 900 RTV
1996 Ford Bronco
2007 Lincoln LT