After Hellcats Over the Pacific shipped, the Parsoft team started work on improving the engine. Tweaks to the graphics engine provided even better performance, allowing more cycles to be spent on other tasks.
These upgrades were released a year later as Hellcats: Missions at Leyte Gulf. Although it was marketed as an upgrade pack, it was actually an entirely new game with its own runnable application. The new version included dramatically improved detail at the airbases, added jeeps and trucks, more detailed ships, the P-38 Lightning and the Ki-84 "Frank", and rockets and torpedoes in addition to the bombs and machine guns.
The rest of the game remained otherwise similar to Hellcats. Both the physics engine and missions system were largely unchanged. The new game included another eight missions, this time with no training. Although they all had considerably more detail and a greater number of in-game targets, they had the same lack of user editability as the original.
Source: Wikipedia, "Hellcats Over the Pacific", available under the CC-BY-SA License.