I know Sneak attack can be applied to ranged touch spells that deal damage.
1: Am I correct in saying that spells bypass damage reduction?
2: When it comes to applying sneak attack to spells, is the extra damage subject to damage reduction even if the spell is not?
3: Am I correct in saying that sneak attack damage remains untyped precision damage, regardless of the type of damage the spell dealt?
1. Most spells bypass damage reduction. Touch attacks do, but not spells that call out physical damage types (piercing, etc.)
2. The sneak attack augments the original damage type. It's all lumped together and either applied to DR or not depending on the damage type.
3. It is added to the original damage roll. Your Acid Splash sneak attack is acid precision damage and your crossbow sneak attack is piercing precision damage, etc.
1) depends on the spell, spells that deal bludgeoning, piercing or slashing damage are affected by DR.
2) It the spell is subject to DR then sneak attack will be, also if the spell is subject to energy resistance so will the sneak attack damage.
3) No it is not. Sneak attack damage is the same type as the base damage of your attack method - acid splash would do additional acid damage for instance. Sneak attack damage is also precision damage, which means that it comes with it's own additional rules. precision damage is not, in itself, a damage type.