I'm currently using four Tachyons (I prefer all-or-nothing loads instead of mix and match) on my Centurion, with two more tachyons in the rear turret, and FF missiles.

How effective are the plasmas in the turret? I've already played through the base game and Righteous Fire once, and the entire time, I've gotten exactly one turret kill (I believe it was with Tachyons). Mostly, the few times I did use the turret for something other than tractoring cargo, it was to deter enemies I didn't feel like dogfighting, rather than actually killing them.

I've always run my Centurion with two Tachyons on the inner slots, and two Neutrons on the outer slots for more oomph when needed. These things are slower than Tachyons, but deal (more than) double the damage over the same time. I usually open up with a full barrage, and then switch down to Tachyons only if they survived that and are too squirmy to hit. And for a bit of seasoning, I've got some FF missiles with that, in 2 launchers.

Using the Neutrons instead of Plasmas, I can also keep shooting longer - and Neutrons do just a tad less damage over time than Plasmas do, but are more likely to hit. Even if I'm chasing after an enemy, Neutrons are quicker to catch up and hit than Plasmas.

On the turret tho, I've got a tractor beam and two Mesons. I wanted something with a high firing rate for curtain fire defense, and I had Tachyons on there originally - but the Mesons are almost identical to those (just a bit less damage per shot) and are white instead of red. Most enemies are using red lasers, so the Mesons allow me to distinguish between enemy fire and mine, allowing me to switch back into the cockpit when I see a whole lot of red incoming and would rather avoid taking that.

I'm not using Shield Lvl3s, to stay agile with more energy for weapons - I tried upgrading to Lvl3 Shields but the Energy lost was quite substancial. I could have switched shield levels all the time as I think that frees up energy, but nah.

... I put quite a lot of thought into that, I guess. And then, the Steltek came along and ruined my setup! I though about putting that on the turret, but nooooo, that's fixed to slot 1. So it replaced my left Neutron. THANKS, STELTEK. Appreciate ya.

*Edit: I just fixed that. 'cause, why not. Now the Steltek's on my Turret, as I always intended it to be after completing the story. Yay! OCD satisfied.

4 Tachyon Cannons in your Centurion should melt everything the game might throw at you. Just be sure you have your Engine Upgrade always 1 level above your Shield Upgrade. A couple Meson Blasters should be fine for your turrets.

Centurion with quad tachyons in front and dual lasers in back. The lasers have the longest range in the game, I've chipped away at many baddies with them. I use dual missile launchers with IRs and just double-tap any enemy I don't want to see.

I actually found dual outer mass drivers and inner meson guns more fun, but much harder. It had a good look and feel.