The Siemens fuel cells are also safer.The biggest problem with AIP now is that subs cannot go beyond 5-7 knots to achieve endurance.So the Stirlings can give you 15 days but at a speed of 3-5 knots.
Being only about 1,600 tonnes,they can hold only about 20 weapons,which is small by American standards(though a US boat weighs nearly 4 times that much for less than twice the weaponry!!!).Anyway,The Russians recently unveiled a conceptual version of their Amur-1650 class AIP submarine for the Indian navy which had an 8 cell VLS for long range cruise missiles in addition to 18 torpedo tube weapons.That's a pretty novel concept if you ask me.
Well, actually that sounds like their adapting the idea from our implementation with the tactical conversions of the four Trident Subs into mere "conventionally armed" ships. They are retrofitted to carry 4 VLS tubes in each regular launch tube space to launch non-nuclear Tomahawks.