It's not breaking because of oil temp issues, so you're wasting your time and money fitting on oil cooler, it's breaking because the Ford planet gears are a weak link and can't take the abuse of hard launches etc. If you want to cure the problem, you will have to fit a better diff....

I run a Quaife ATB in the back of mine and have had NO issues what-so-ever. I do 4-6 track days plus two excursions to the Nurburgring every year. Obviously it still uses the OE crown-wheel and pinion (like all the replacement diffs do), but this isn't an issue if the car is not launched everywhere (my Quaife has been in the car for three years!).