I'd argue a 27:04 in Atlanta on July 4th is better. I looked it up - the low that day was 61 and high was 97.
A 27:04 on a net downhill course in a drizzle (the high and low temps alone that day don't accurately describe the conditions for the race itself that day; at 7 a.m. do you think it was closer to 61 or to 97 degrees?) is better than a 27:52 at over a mile above sea level on a rolling course that gains about 75' from start to finish? Maybe if Kimani had been wearing chain mail.
Granted, the Peachtree course loses 300' before gaining about 200', so it's not like it's an easy PR. But on cool days, people run fast there, the problem being that Atlanta is virtually always hot and humid in July.
Both performances are fantastic, but only Machuka's is otherworldly.
http://www.peachtreeroadrace.org/documents/download/2012ajcprrmediaguide (See page 35)