If you look at the Battle of Okinawa, you'll see that these guys were still well able to dish it out, even this late in the war. This is not the Volksturm in Germany late 44 and 45.
A determinant factor was largely whether the local Japanese commander in charge would be dumb - suicide charges - or clever - anything but suicide charges. At Okinawa, one officer (Yahara) was recommending attrition and harassment, another officer (Cho) glorious and fearless attacks. The Japanese did fairly well, i.e. did not collapse, until the first lost the argument and they reverted to banzai mode. Yahara, IIRC, wrote a book about the mess afterwards.
Same thing at Iwo Jima, a bit earlier, with a commander unwilling to waste his men, but willing to sacrifice them to inflict maximum US casualties.
The garrison at Iwo Jima was typical "2nd raters" as per this logic as this unit was reformed in May 1944 from what seems to be non-veterans. Basically, while this is an interesting question, I think that what might be a massive factor with green Western troops - morale and breaking or not under fire, just did not apply as much to Japanese troops, who would not break but were outgunned and often wasted by their officers.
China veterans may been less willing to forego heroism, believing elan would win the day - which it might do against more limited Chinese firepower. Or they may have been more clever and cautious instead. It is hard to know and one would expect variation from individual commanders, along with extra resistance due to invasion of the homeland.
The comparison with pilots can be a bit misleading. First the Zeros started out technically superior, with some flaws, but were gradually just plain outclassed. Second, the Japanese Navy apparently was apparently not good at mass training. They had excellent pilots at the start, but once they were killed, no one of the same caliber replaced them. Infantry just requires a lot less training than pilots.
The civilian casualties in Okinawa were horrific btw.