VERY good questions.
My take on what I THINK I know:
1. "Does quench matter"/affect power? Yes. But helping to reduce ignition advance requirement and allowing an engine to run a higher cylinder pressure when octane limited without detonation. Achieving an appropriate max squish velocity at peak HP RPM is the goal and quench is one of the variables impacting it.
2. "How much?" An advanced sim that takes into account max quench velocity and calculates ignition advance requirements take that as one of the inputs could, on a given engine estimate how much less ignition advance is required with best-case quench vs. worst case quench... In HP? -Not a clue; quite interested in a potential answer to this one. (Seems like an answer in HP @ HP PEAK RPM would involve a LOOOOOOOOOT of variables.)
3. How much quench clearance is enough? Enough to get the max squish velocity at the desired peak HP RPM to 15-20 m/s on the engine in question-according to Blair; 15-30 m/s according to TSR, I've seen David Redszus quote 25-30 m/s in one place and I think 25-35 m/s in another thread.