Not necessarily (if the data exists then this paper would already have referenced it, surely? These are serious scientists at very respectable institutes - the INT at Seattle is highly prestigious). Firstly, you would need to select rays that are very close to the cutoff...the further your distribution slides down the energy axis, the less noticeable your deviation will be. Secondly, you need to orient your distribution geo-spatially, i.e. take into account the Earth's position throughout the measurement. Thirdly, you need to find a way of correcting for all known sources of very high energy cosmic ray background.

I'm not sure whether anyone has ever done all that at once. I'm not saying that they haven't; I'm just not familiar with any measurement that meets all those criteria. I'm not a cosmic physicist though, so it's entirely possible the measurement has been done.