In this case, the info should be edited
Compared to your current Sport wheels, 305 tires, and 20mm spacers...Nismo wheels would sit like this:
-No spacer and 285s means they sit 23mm further IN.
-No spacer and 295s means they sit 18mm further IN.
-No spacer and 305s means they sit 13mm further IN.
-No spacer and 315s means they sit 08mm further IN.
-No spacer and 325s means they sit 03mm further IN.
So...if you're running 285s on the rear Nismo wheels at first, you can leave your 20mm spacers on and your tire will sit almost the same as your current, 3mm further in actually so it should clear without issues.
If you put 325s on and remove the 20mm spacers, it'll be the same 3mm further in. So, ideally if you want a slight poke, you should install 5mm spacers in the rear when running Nismo wheels with 325s.