I always assumed that the bottlenecking at IB was to give horde time to summon their Icelord (in our battlegroup, SF is always ceded to horde to give us a clear run to FW). The farming creates a huge supply of the turn-in mats and the horde summoning point is very defendable. That said, since the patch I've yet to see Icelord summoned in our battlegroup (30+ AV's).
I agree with other posters - horde's main downfall in AV is lack of defence and lack of strategy/co-ordination. 9 times out of ten, FW is completely undefended and gets capped by 2 players. There are 3 routes to SP (hilltop, under bridge, and main path) - never seen horde try all 3 at once to break up allies defence.
Another point: horde complain that the SP road is a chokepoint - look again.. the main hill overlooking the flag is sloped so attackers can control it and snipe the hell out of defenders around the flag (one of the main reasons to get extended range talents on my hunter).
The only AV losses have been when no-one defended SP (apart from me, but I just got creamed by 20+ horde), and then continued to ignore the attack on base from there on.
Another tactic which threw a winning battle off course was sneaking in the back of Allies base - hardly anyone knows about this tactic it seems (tactic, or exploit? subject for another thread). 1 horde player can cap ally aid station while everyone is fighting at SP.