The answer is in the Torrent rule itself:
TORRENT
Torrent weapons shoot clouds of fire, gas or other lethal substances that few foes can hope to evade.
Weapons with [TORRENT] in their profile are known as Torrent weapons. Each time an attack is made with such a weapon, that attack automatically hits the target.
But for kicks, I'll add in the Rules Commentary things that support this:
Automatically Hits: If an attack automatically hits, the Hit roll for that attack is automatically successful (see Automatically Successful).
AUTOMATICALLY SUCCESSFUL
When a dice roll is automatically successful, the following applies:
■ If a roll is automatically successful, do not roll the dice. Any rules that take effect on a particular dice result or roll result do not take effect if the roll they refer to is automatically successful.
■ If a Hit roll is automatically successful, move straight to the Wound roll. Such a Hit roll is not a Critical Hit, and does not trigger any effects that are triggered by a Critical Hit.
■ If a Wound roll is automatically successful, move straight to the allocation of that attack. Such a Wound roll is not a Critical Wound, and does not trigger any effects that are triggered by a Critical Wound.
So there you have it. Torrent weapons automatically hit, which means they never roll to hit. Only hitting on a 6+ is therefore irrelevant to Torrent weapons.