Don't forget it is likely you could be exceeding the maximum speed for the disk.
I totally get the whole use every bit ethos but cutoff discs aren't expensive.
Definitely - air tool may well run well over double the speed of an electric grinder!
And if you are like me, bulk bought cut off discs when the price was good, you are using discs that are pre-expiry date - so they disintigrate as they spin up on a battery grinder!