In the case of the holder of an IR issued in accordance with ICAO Annex 1 (e.g. an FAA IR), JAR-FCL 1.016(a) states that he "shall meet all the requirements of JAR–FCL, except that the requirements of course duration, number of lessons and specific training hours may be reduced." i.e approved training is still required but the duration may be reduced.
Do you have a reference for how much it may be reduced? There I suspect is your answer.
Why do some people have such mental blocks on the concept of
demonstrated competence? You
passed the checkride!!
Why fly extra hours? Even if you do fly the extra hours, if you have reached checkride competence early, any honest instructor will try to not waste your time and money and maybe will do some other useful stuff which is not required for the IR checkride but which might come in useful.
Anyway, unless you work for the CAA (do you, BB?) who cares once you have the IR? A few months later, the new IR holder is only as good as his last flight. A few years later, all of the original flight training has gone right out of the window.
The theory exams go out of the window within a few months at most.