Did a quick bit of reading from the IS owner's handbook and from other forums and it would appear the ECT PWR button may also sharpen the throttle response, like Sport mode in my car. Still not sure how, once off the throttle it could affect how the car 'strains' against the brakes?
That's not to say your observations are questionable, just I'm not seeing the link. Hopefully others more knowledgeable will chip in.