A uboat is a very poor AAA platform.
They did experience some successes against aircraft, however the number of uboats sunk by airplanes far exceeds the number of airplanes shot down by uboats.
If you want to upgrade your boat you'd be wise to use the renown on some of the more passive defenses like bold decoys, anti sonar coatings etc.
If you choose to fight it out on the surface with planes... Especially in GWX you might experience a couple of successes, but for the most part you probably won't last long.