The 1500 meter limit isn't a good comparison of the 338 Lapua to a 300 Win Mag. Both are adequate out to a mile with milspec loads (220gr SMK in the 300 Win Mag, 2,850 fps). Right now USSOCOM is looking for a 2000 meter objective sniper solution. A 338 Lapua pushing a 300gr SMK at 2,800 fps stays supersonic 300 meters past the 300 Win Mag load, and has the same drop at 2k that the 300 Win Mag has at 1850 meters (roughly), and a half second faster time of flight out to 2k.
Note that neither the Lapua nor Win Mag will remain supersonic at a mile (at least not with the bullets or loads tested), but choosing bullets that handle the transonic shift allows use beyond that limit.
So the Army does care about "shootability" for the next gen sniper rifle. But the range capabilty is looking for 1500 meters are the "interim solution" with an objective (final capability) of 2000 meters.
Of course NSWC Crane put together the 220 SMK 300 Win Mag load to compete directly with the 250gr loads our allies are using, and it does so quite well.