Believe it or not, you won't see a huge increase in HP. Unlike a 5.0 Mustang, adding long tubes to the Lightning just doesn't "wake up the engine", like it does to the Mustang. That's because the stock exhaust on the Gen 2 Lightnings breath pretty well in stock form, and it isn't until you are generating quite a bit of HP over stock that you see more than an incidental increase in performance with the addition of long tubes.
As for sound, you'll have a marked tone difference. Long tubes produce a deeper, more robust sound. Volume will increase also, especially if you choose a no-cat setup, straight back to the mufflers.