Extra powder is an interesting mechanic in principle, but dull in its application with no counterplay.
It makes vampire missiles stronger at alpha striking & the first part of a battle, which is odd for a slow & lumbering faction.
The bonus will always apply as there is no way to prevent those first few shots, unlike the high elf bonus which can be played around to some extent.
It makes using the wight's ammo ability dull to use.
The ability would more interesting & thematic if reversed - the last few ammo shots would deal increased damage.
This incetivises protecting missile pieces even more and feeds into what undead do best - outlast their enemies, not alpha strike them.
The enemy will have to be extra careful in prioritising what units to remove first before the bonus kicks in.
To balance ranged summons, their last shots before desintegrating would have this bonus.
Wight's ability can be changed as well.
The bonus can be mode stronger than it is now due to it being conditional - most other factional bonuses in the game are conditional in one way or another and can be partially negated by a smart enemy.