poor combustion,poor fuel eficiency,poor performance,pinking can occur,ecu is going to advance the timing,wich can make the car s performance bad on different ranges,you might not see the diffenrence,but its there,for long term,its not reccomended,why?
poor combustion makes the fuel to not burn right,so less fire=more carbon in combustion chamber and on the valves,o2 sensor gets dirty faster,catalytic converter gets dirty,egr valve getting stuck....
your car already have egr troubles like every other proteges,they all have....
dont run after troubles,you can try them in,but as an advice,put the oem recommended number....
a thing you can do,with the same number,you can change to platinum,or even iridium,wich will last longer,and be safe for your engine...