yeah, if you're willing to lose turn rate, acceleration and top speed for an extra pair of guns, "buff" those ships with some extra cannons
I also think it would be completely absurd if pirates could craft any ship in the game that nationals can, plus the Pirate Frigate.
As for Basic PFs - they're garbage for boarding. They get some extra prep? With 0-2 upgrade slots, big deal. I take a ship with 5 upgrade slots, put basic marines on them and get far more out of this deal than the inherent boarding bonus of the Pirate Frigate. There are just too many good boarding mods out there to build a basic ship for boarding
Axes - Ladders - Marines - Boarding parties - Grog rations - swordfighting handbooks - hammocks. That's 7, and you can only pick 5 with an exceptional ship.