I've never heard of anything out of the 270s
I don't know if there actually is a theoretical "perfect score" or whether that's just a myth. Based on the scoring system I'd guess myth--its standardized to make the mean ~215 every year, so the top score possible should change every year depending on the % correct of the mean.
Example.
If one year (call it 'strong year') the mean % correct is 87% and you get 100%, you're probably not too many SD above that mean and your 3 digit score wouldn't be that high, say a 250.
Next year ('weak year') the mean % correct is 60% and you still got 100% right. This year you're many more SDs above that mean, so your three digit score is higher, say a 280.
So you got 100% right (theoretical highest score) in both 'strong' and 'weak' year, but your three digit score was higher one year compared to the other. Obviously the %correct of the mean doesn't vary that much from year to year, but I did it to illustrate a point--that the top 3 digit score possible should vary from test cycle to test cycle.