OK, it is admittedly hard to sound objective here, but I would advise that those looking to increase HP, you'd be remiss not to also install a transaxle cooler. In fact, in the 2007 version of the hardbound Ford Racing Catalog, they pre-announce their upcoming pulley upgrade and advise that the transaxle cooler be installed in advance of HP increases (and we all know they're as objective on the subject as I am... lol).
Over the last several months I've personally installed more than a dozen coolers and *most* of the cars had exhibited some residual oil around the stock breather. This oil accumulates from the heated vapors escaping from the transaxle - caused by considerable heat build-up within the transaxle. Adding and subsequently utilizing more HP, will only add to the heat absorbtion within the transaxle - hence Ford's recommendation. Although the aftermarket breather kits are also a practical accessory, the transaxle coolers also eliminate this breather leakage by addressing the root cause.
On the positive side, the presence of a transaxle cooler does add some bling for your under-hood admirers in addition to adding some peace of mind for the well-being of the car.