I bent my Ultegra derailleur (it's cage is approx 5.5mm). Can I replace it with a Ultegra derailler with a longer cage (approx. 8.5mm)? Don't know if you need this info: two chainrings and a nine speed in the back.
Yes, the longer cage will work. I think your chain will be fine, too. Isn't the formula: big-big plus two links? That doesn't depend upon the cage size.