The wife has been rearranging the house so I'm having trouble finding the book...lol! But the statement is in it.
I can only assume that against the opponents that Hellcat pilots faced in 1943 and 1944 the 10 to 15 mph of extra speed that the WEP units delivered weren't really worth the extra 500 to 600 pounds of weight they added to the F6F's airframe. The empty weight of the -5 Hellcat was 9153 pounds. Subtract 500 pounds from that weight when the water-injection system was deleted and the empty weight dropped to about 8600 pounds.
According to Corky Meyer, a factory-fresh F6F-5 with wep had a top speed of 409 mph at 20,000 feet. Tests conducted by NAS Patuxent River which compared the performance of the -5 against a Zero 52 corroborated Meyer's figures, giving a top speed only 4 mph slower than the F4U-1D at their rated altitudes of 20,000 feet. That top speed was 74mph faster than the Zeke's best top speed of 335mph at 18,000 feet.
So the extra performance of the water-injection unit may have seemed superfluous to the Navy pilots who flew the Hellcat. The lighter Hellcat, sans water-injection, would have been more maneuverable at all speeds. And maneuverability would have definitely been appreciated by any Navy pilot caught low and slow against lighter Japanese opponents.
Regards, Shuckins