The old Leech definitely has an effect in FSPort ST/ST:R, at least on Hard; I definitely find it makes Valks and Ulysses less AB-flighty, thus making them easier to pin down with the relatively slow-moving old/original Prometheus(otherwise, I'd be using nothing but Avengers).
FS2 top-drawer primaries(Prom-S/Kayser) are too powerful to really justify using 'tactical' guns like the Lamprey and Circe in single player(at least, in their lackluster states as they are). There might have been justification in FS1 where the Avenger and Prom had shield damage-over-time values on par with the Subach, but the Prom-S and Kayser are so brutal on both shields and hull, with excellent shot-velocity(and are still relatively energy-efficient), that you honestly don't need anything else(barring subsystem/turret work, which the Maxim is custom-made for).
It definitely doesn't help that the Main FS2 campaign gives you access to the Lamprey and Circe only after the Prom-S is (re)introduced, as they might have had a short period of semi-relevance in being paired with the relatively low-power/output, equal-shot-velocity Subach.