To me the Y-15 ,seems like what An-70 was trying to be, since the main limitation in practice by frontline airtransport is volum limited and not weight limited.
You would run out of space first before you hit the 20t weight limit. That why the C130J-30 exists.
So making a bigger volum plane is more critical then it's payload weight.
But then again we don't know what the bigger picture is , capability wise I am not sure that PLA consider the Y-15 ability to carry type 08 or type 04A for 1000KM to be critical, but it probably would want a transport that is capable of supporting Light-CAB .
Using the US numbers as a baseline here , an Infantry-CAB would need at least 40t a day to fight ,60t if you want optimal, so for the Y-15 that 2-3 sorties, so I think the PLA will be happy as long Y-15 can fit the Type 161 with it's ammo fly it for 1000KM and get it on to a dirt road operationally.