I didn't notice an improvement in power after getting the pipe replaced, but my car is running badly at the moment anyway so that's not surprising!
I paid a garage to fit the pipe at the same time as they did the clutch so not sure how difficult it was. Think it only took them 30 mins or so. You'll need some new jubilee clips as you have to remove some of those "Use only once" metal clips holding the existing pipe in place.