A later model, the A6M6c, with a different engine, had WEP. The AH3 model is correct.
+1 on this - Mitsubishi Zero: Japan's Legendary Fighter (book by Peter Charles Smith) confirms this as it went in great lengths to explain the variants and improvements with each model. Couple of points:
1. The years of service of our A6M5b means it was a pre-WEP model
2. The IJN had issues getting it to work properly until early 1945
3. Ours is a August 1943 variant, and by then, the IJA and IJN were in deep trouble producing anything, let alone developing the performance of their aircraft. And even then, there were not too many WEP models out there - if they worked at all.
Page 163:
Engine boost systemWhile the Zero herself had been steadily improved, lack of a really powerful engine continued to restrict just what improvements could be made to the existing airframe, which these additions had now increased by 700lb (3.175 kg) additional weight with the same power plant. Mitsubishi required the Navy air arsenal allow them to fit the more powerful Mitsubishi
Kinsei-62 radial to this aircraft, but the request was rejected outright. The Navy's solution was to insist on fitting the existing Sake-21 with a water-methanol injection system to boost its output for short bursts during combat.
21 However, such was the slow pace of development of this system that the HEI had to go into combat without it. Eventually one trial aircraft appeared late in 1944 fitted with the
Sake-31a radial. It was far from successful prototype; the injection system was still in its experimental stage and was plagued with continual problems, while the modified engine generated
less power not more. The A6M5c sub-type was built at the Nakajima Koizuma Plant, and attained a maximum speed of 346mph (556.83km/h) but only ninety-three ever appeared before the whole project was cancelled in the increasingly frenetic situation the Navy found itself that winter of the war. Production was complex and still mired by problems, maintenance of the temperamental power plant in the field difficult.
Work on the water-methanol injection system continued, however, utilizing the Nakajima
Sakae-31 engine, which was a fourteen-cylinder, air-cooled radial, which was rated at 1,130hp (831.11kW) for take-off, (1,100hp (809kW)at 9,350ft (2,850m) and 980hp (720.79kW) at 19,685ft (6,000m).