Supposedly Powell ran 6.31 in Lausanne (slight altitude). Coleman split 6.32 in his 9.76. Greene 6.33.
Ben Johnson also split 6.33 and Gatlin 6.34.
Indoors has no wind, so the official 60m times come with 0.0 while all but Greene's of the above have tailwinds.
Depending on how you count the times (Bolt had 6.31 with cameras sharpened to 6.29 with lasers while I think the others used only the camera method), Powell or Bolt has the fastest converted 60m split after wind and altitude, 6.32.
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Some other interesting stats include the fastest 10m split ever, Bolt 0.80 during his WR. (Supposedly Gay ran 0.81 in that race.)
The fastest converted 100m time might be Bolt's 9.77 -1.3 with a 0.223 reaction time (9.69 wind, 9.61-9.62 with a realistic r.t.)
His 2008 Olympics WR where he pounded his chest at the end might have a similar conversion, if you can figure out how to convert his slowdown.