Kind of dead thread, but I agree.
I've generally got about average performance - I work on getting two spawns, more or less, per tervigon. Which is still about half the points value of a gunned-up tervigon's worth of gaunts.
More importantly, is it worth it post-spawning.
In my view, yes. 6, T6, 3+ armour, Feel No Pain wounds is next to impossible to shift without beast-hunter shells, force weapons, or other instant death shenannigans.
Tervigons are the single most durable thing in the tyranid army, and whilst not great in combat for a monstrous creature, you must admit that's a bloody important qualifier. Smash has left them quite able to total characters, tanks and bastions with scary ease, and the area effect toxin sacs, adrenal glands and counterattack is worth its weight in gold. I've seen theoretically elite assault units vanish into a tidal wave of lil' bugs - 60 poisoned attacks takes down anything.
I agree that at least one or two big broods are worth it. The theoretically legal ten per big bug works but a couple of big waves to make contact without getting shredded and provide some protection to the spawnlings helps immensely. Plus they're cheap and fearless, so it helps immensely in providing a screen between your soft-and-squishy units and canoptek wraiths, death company, and other dastardly Ward-spawned nightmares.