According to memory alpha...(not cannon I know) the Galaxy Class was launched in the (late)2350s. The Akira class was Launched in early 2370's. so thats nearly a ten year gap. Starfleet became aware of the Borg in 2365 (memory alpha), so the ONLY reason I can see the 15 torpedo tubes reasoning is that the Akira fires a Torpedo Orgy basically at a borg cube, hoping for a knockout punch. possibly working in concert with defiant class ships. still seems over the top but thats the only reasoning I could find behind such a large amount of tubes on a medium sized ship. considering most other fed ships have 2-3 tubes that can fire in bursts, it sort of renders the whole broadside cannons thing moot. its like replacing a machine gunner with a WWI rifle line.