ResExsention To add on to this, I'm going to try and make some details about the pd to make it more unique, if thats alright.
Cons:
Adding on to the fact that it's a gun with a slower traverse speed and fire rate, it'd also have a limited amount of firing angles, though not to the extremes of the flak cannon. The HPD would also have a very high fragility, due to something I'll delve into later. The HPD would also be very explosive if hit when "fully loaded". Moreover, the later topic, when fully used, could result in a "cooling period", where the gun is turned off for a few seconds. The topic can result in the gun actually outrunning the power being fed to it, leading to an early "cooling period".
The Pros
The HPD, for it's cons, are made up for by a few things. For example, The HPD would have a longer range than even the Flak cannon, and with it's dual barrels, should be able to target missiles from farther away. The shield damage from the HPD would also be acceptable, meaning combos of HPD and Flak could break down Shields if given enough time. However, the major feature of the HPD would be one thing:
Overclocking
This is the major selling point of the HPD. For a duration similar to the booster thruster (though it's a fixed time and cannot be extended past maybe 7 seconds), the guns will consume more batteries than usual. For this brief period of time, the HPD will have a fire rate over 3 times the amount of the smaller PD. This is also helped by the fact the HPD will do MORE damage to enemy missiles and enemy ships. The HPD will, immediately after it runs out of energy or goes past the time limit, will enter a "cooling period", where it cannot be fired, rendering it useless.