I've heard similar things but I don't believe it is all that accurate to be honest. ECU's are designed to adjust to how sensors run rather than how the driver drives the car. You might notice some difference though, as you say, by 'resetting the ECU'. After you've left the battery disconnected, idle the car for 5mins or so until it's warm, take it out, driving slowly at first, then put it through it's paces. People don't generally tend to do this to make their car run 'faster'. more as a resolution to problems with electrical sensors on the engine.
ECU's also log faults which can be detected when put under diagnostics tests at garages. I think with most cars you can have these faults erased when you have a diagnostic test done which can free up memory and help the ECU to respond better, but generally wont boost performance a great deal.
Obviously cars have different ECU's and ECU's have different manufacturers so it's difficult for me to say. I'm sure James will correct me somewhere in this post!
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