A lot of your hypothetical hinges on GS / MS buying at 15 and holding at prices below 24. Which is a lot like saying they actually value it at 24, but were somehow allocated shares at a bid lower than that. IMO, Google did it right with the Vickrey / second price auction, and apparently this has been formalized OpenIPO: into https://en.wikipedia.org/wiki/OpenIPO
If we assume that GS and MS have a reserve price of 24, as evidenced by the market open, then their strategic 15 and 16 bids do represent money left on the table.