I've looked at the fiche quite a bit, and as expected there are superceded part numbers every year. The general assumption can be made that 'newer is better', but that is not true simply on the basis of a changed part number.
Case in point: The engine cases have different part numbers for 2006-2007 (2S2-15100-01-00), 2008 (2S2-15100-20-00), 2009 (2S2-15100-21-00). Is there really any difference between them either from a performance, or internal parts interchangeability perspective?
Also the transmissions have quite a few changes in gear and shaft part numbers, but parts appear to be functionally interchangeable, including gear, shafts, shift forks, etc.
Could you throw multiple year trans parts into a box and select the needed parts at random and they would fit and work properly?
Thanks for sharing what you know.