Up The Ante raises the ammo capacity of every ranged weapon. It is a Tier 5 Ranged skill that matters most for Horde fights, where running dry is the usual way to lose.
Yes for Horde hunters. For general play it is a nice bonus rather than essential.
Two Birds, One Bullet, Iron Grip and the Drifter Bike's Storage upgrade.
Park the bike near a Horde fight anyway. Extra carry space does not replace a refill from the saddlebags.