To be blunt, it's supposed to be hard
. Top aces generally did not score more than once per mission, multiples were cause for celebration.
To quote MvR: "Aim for thr man, not the machine". This means getting to point blank range and hosing the cockpit. It's still not easy but a single bullet to the back of the pilot's head is guaranteed to bring down the plane
. Once you start getting head shots your score will rise.
The reason for the increased difficulty is the DM. I think HA uses Zinteck's new DM, as does the new WFP. PL is a graphics/sound patch and will not effect things like FM or DM when used with another UOP like HA or WFP.