I don't think there's a possible highest cap in USMLE scores. If we hypothesize that they follow a statistical distribution curve then any given score should have certain percentile above it. Learn more about
USMLE Score Calculation.
Again, if we postulate the distribution curve and the fact they give a mean and a standard deviation in score reports which are currently around 220 for the mean and 20 for the standard deviation then a score of 280 should merely indicate three standard deviations above the mean which means still you have 0.3% of people above it!
Wikipedia article talks about a maximum of 300 but I don't think that's correct.
The highest score that I personally heard of is 272 in CK.