There is a theoretical potential for extra rotor cooling. But I suspect
someone just bought 17" wheels and is now unhappy that the stock rotors
look lost in all that empty space. There was no mention of a problem to
be solved, and stock Miata brakes work pretty well.

I'm still trying to figure out what the heck an "NBFL" is? Is the
front-left disc bigger than the others? Are NB brakes larger in Florida?

My problem has more practical manner. :) I ordered slotted&drilled rotors
for NB Miata, but delivered rotors are for NB-FL (Face Lift) or NB with
sport package.

So, they are a little bit to big to fit them on my 2001 NB 1.8 Miata. I have
two options now:
1. Try to sell this rotors and wait month or two more to get the right ones
(I live in Croatia, and I bought rotors in California, so option to return
rotors isn't very practical because shipping cost me more then 200$)
2. Somehow fit this rotors on my car (it seems that calipers can be fitted,
only mounting brackets are different)

Yes, I admit that bigger rotors are little bit heavier, but the cooling will
be also better.  I'm not stratified with stock setup on Miata, because after
4-5 laps on demanding track with heavy braking or longer downhill drives,
brakes are simple unusable because they are overheated. This is my second
Miata, I also owned 2001 NB 1.6 and in last 5 years tried many performance
brake pads (Ferodo, Mintex, EBC), breaking was a little better than with
stock pads, but overheating was still there.