Oh, and i doubt items can be "changed" depending on fighting in Battles or Sieges.
So yes, some builds/items are more effective on sieges than others, but seeing as the item date is the same for both, you can't change them (i believe).
And... if you can see my earlier post, it was "quoting" someone, and answering him... about adding extra ammo with heirloom. Wich is nearly the same as adding a 4th stack if carrying 3 identical stack, except more variety. And variety is good.
Also... my "dedicated"' thrower used to have 6 PS and 50 wpf in 2h. So i'll throw my stuff around, and loot a nice 2h weapon somewhere. That way, i'm still playing/slaying having fun. Just as you see archers with a melee weapon, except that we (throwers) go in melee sooner, and with less malus since we can keep a nice armor.
If you're a fully dedicated thrower (1wpf everywhere, 0 PS, 0 Riding, etc...) then yeah, lack of ammo. Try to find friends/allies who also throw stuff around. And if you can't pick up back any more ammo, then that means you've killed enough people already, and can go berserk with a melee weapon, even with 0 PS. (try the 1h stab)