That raises an interesting question for me, I hadn't thought it through before, but when using the Ion cannon would the one damage be a critical because of Marksmanship. I think not because the damage is the result of the hit not the dice, but I would keep Marksmanship for the primary weapon as it makes sure you give two stress.

No, the damage from the Ion Cannon would still be regular damage: as you say, the hit is the result of the Ion Cannon's text, not from the attack (all dice results, including any crit results, are canceled). However, it does mean that if you miss with the Ion Cannon you very likely assign a stress to your opponent.

I would not use an Ion Cannon on a Firespray. The Firespray is one of the few damage-dealing ships in the Imperial fleet and I would not like to intentionally mitigate that. If you are looking for a way to make the opponent's movement predictable, a Scimitar with Flashette Torps is a cheap way to do it without drastically reducing your damage output. That and Bombers are fairly beefy.

I would not use an Ion Cannon on a Firespray. The Firespray is one of the few damage-dealing ships in the Imperial fleet and I would not like to intentionally mitigate that. If you are looking for a way to make the opponent's movement predictable, a Scimitar with Flashette Torps is a cheap way to do it without drastically reducing your damage output. That and Bombers are fairly beefy.

the OP doesn't list owning a bomber in the sig, but I'll keep that in mind for my own use.

I must say having considered it now I don't think I would take two Scimitar with Flechette torps over that Kath build, even if it is 36 points vs 38 points, 12 hull vs 10 hull.

I wouldn't compare a one off shot Bomber against Kath every turn having a guaranteed stress (alone or with 1 damage/ion) , or the primary's damage plus a very likely two stress with every hit, every turn.