This is a tough question as typically it garners responses that are mostly guesses without any real evidence one way or another. By evidence I mean, someone actually dyno tested the difference any chip might make in power and torque (or can't point to evidence.) Not, it "felt" faster, or "I heard these were a scam", or going off on tangents like "you'll void your warrant" or "why do you want to do that?"
It's widely known stock ECU programming is conservative. If you have a car programmed to take 87 regular octane it's common knowledge you can increase horsepower if you use higher octane fuel and advance the timing (and alter air/fuel mixture curve) as the higher octane fuel will not create engine knock when advancing the timing and you'll get more power.
Never heard anyone claim the $20 chips on Ebay make any real difference, and that would confirm the saying "you get what you pay for." But what about the $100-$200 chips that plug into the OBD2 port? Are they really any different than the cheap Ebay models? I've also seen chips that plug into a sensor under the hood and others you splice in.
Wondering if anyone has found a brand that is reputable, producing products that actually give you some real gain in HP/Torque (like 7-10% or more) in conjunction with running premium gas on the lastest model Japanese 4-cylinder engines from Mazda, Honda, Nissan, Toyota and the like? Someone that's either done a before and after dyno test or before an after with a performance meter like a Passport GT-2. Seems if you can't at lease see a reproducible difference on a performance meter before and after chip, the increase is then non-significant.