After racing with him and seing how he behaves on track, I would most likely say it's him... If not he is a very good iracer who just wanted to make a joke... I dont know many people that can jump into the MX5 and end up 0.3s slower than me @ Lime Rock... He could though and he was racing really well and cleanly...
So he might not be THE Lewis Hamilton, but seing him up online just a couple weeks after he allegedly used iRacing to practice on Watkins Glen for the driver swap with Tony Stewart does seem rather plausible to me...