the whole chief firing the howdah weapons argument has been a pretty hot debate, I'm not sure a definitive answer has been reached yet.
IMO the venom is pushing it
I agree with bat Manuel, the spear's the way to go.
2d6+1 S6 impacts, followed by 6 more S6 attacks. (3 from the steg, 3 from the lance wielding chief). Averaging 9 S6 hits will break almost anything, let's be honest. Add a slann w/ lore of shadows (unseen lurker) or beasts (wolf hunts) and life gets HILARIOUS. 12" march up a flank, then 12" charge into said flank, and start rolling up blocks of units. Kinda pricey at 380, but I think I'd rather have the spear chief than a 2nd engine of the gods...