In the past EC2-based clusters have made the Top500. Somewhat unique then because they were the only virtualized systems, but those were just 10 gig ethernet and HPL runs really well on those. In all cases we did it on relatively small number of machines before they had been publicly launched (essentially we used HPL as a stress test).
Look at the % of peak for ethernet systems vs various Infiniband speeds, or Intel Omni-Path, and you'll see that there's a significant different in the % of peak for Ethernet vs Infiniband vs Omni-Path.
(Work at AWS)