degobah...Lando here....sorry it took so long to reply. I started a reply last night, but then an internal aggregation switch in our network froze and I had to take care of some ****.
A pitch stop mounts on the top of your engine and against the firewall to stop the engine from rocking, or pitching, forward and back under hard acceleration or deceleration. Your car has one stock, but the rubber pieces are soft and gummy, and normally contain gaps in the rubber to further soften the link between the two parts. This makes them function more to dampen the vibrational noise than to stop the pitching effects of the engine.
Subaru makes an upgraded model, known as 'Group N'. The pitch-stops, motor/trans mounts, and bushings in the Group N class are factory performance parts that fit into the strict qualifications of an Australian rally division of the same name and are a level between standard stock parts and full-on race parts. The Group N pitch stop has stiffer rubber than the stock piece, and normally no voids in the rubber, making them perform better while still helping to reduce vibrational noise.
Both the factory standard and the Group N pitch stop are comprised of soft rubber and a composite shaft.
Aftermarket pitch stops are normally made of aluminum shafts and polyurethane bushings which are far stiffer than rubber, though there are some full-on race parts that are solid aluminum. The pitch stops that contain polyurethane bushings greatly reduce the pitching of the engine (almost completely) though you will notice slightly higher vibrational noise.
So, what does all this mean for your car? With a stiffer pitch-stop the engine will not tilt while accelerating, which will increase the responsiveness as you will not loose torque from the engine shifting around. This helps more of the power you are already producing to get to the wheels.