57" Hg was for the high blower ratio engines (9.6:1) of the P-40M/N (and P-51A, P-39N/Q) which resulted in detonation at lower boost levels than the 8.8 blower of the other allison powered P-40's.
The 9.6 resulted in a limitation of 57" Hg with 100 octane fuel, which was faily close to the detonation point. Limiting the engines to 1,480 hp, but allowing this to be acheived at 10,400 ft, due to the higher blower ratio. (giving the P-51A a top speed of 415 mph, P-39Q-10 397 mph, and the P-40N 378 mph)
The 8.8 blower was rated for 60" Hg with the same fuel, giving 1,570 hp with detenoation still a good way off, but limited due to structural concerns. Higher boost levels were used in service sometimes, but risked structural failure, as the engine's gearing wasn't rated for more than 1,600 hp. With 66" Hg resulting in 1,770 Hp at 2,000 ft.
(similar to the AVG's Tomahawks overboosting -and overreving- to well over 1,200 hp on the -33 engines, while the engine itsself was generally strong enough and there was no porblems with detonation, the gearing on these early engines was rated for only 1,100 hp, and thus would tend to strip eventually)
See:
Perils P40 Archive Data
http://www.raafwarbirds.org.au/targetvraaf/p40_archive/pdfs/Allison 1710-39 abuse.pdf
The two things the Allison people were concerned about with such overboosting was firstly, the risk of structural failure, and likely at some time after WEP was used, in normal operation which would make a very bad situation, particularly if over enemy territory.
The second, more pronounced concern, was that resetting boost to higher levels on the 8.8 blower engines would result in a bad precedent being set which would result in many failures of the new 9.6 blower (-81, -83, -85, -99) engines as it would result in detonation (that being the primary limit to 57" Hg -1,480 hp at 3,000 rpm and 10,400 ft) and failure quickly folling that.
It should also be noted, that early on the P-40D/E's V-1710-39/F3R engine was rated for 56" WEP (before being cleared for 60" allong with the similar -73/F4R of the P-40K). At that rating 1,470 hp could be acheived at ~7,000 ft with ram air. (~5,000 ft w/out)
http://www.raafwarbirds.org.au/targetvraaf/p40_archive/pdfs/1710-39.pdf
http://www.raafwarbirds.org.au/targetvraaf/p40_archive/pdfs/1710-33.pdf