I am a little surprised at the lack of success with the Firespray. I actually wasn't sure I was going to buy one, but was glad to win one at a Kessel Run. I have had a blast playing with it and also been really successful.
The only time the Firespray has been awful for me has been when I have overspent on it. As long as I keep the upgrades that I use low, then I have no trouble doing well. In fact, I am already planning lists involving 2 Firesprays.
It may be that they suit my playstyle. With wave 1, I typically played 3 ship Rebel builds. While those are seen as weaker, I did make it to the finals of 2 Kessel Runs with 3 ship Rebels.
Anyways, I when I play the Firespray, I primarily play Krassis with an Ion Cannon and a Seismic Charge. Then I sometimes add a Gunner or Missile, but just as often not. At just 40-45 points, I can easily add 4 TIEs to come along and support him. Krassis's ability means that he can evade every turn and still have a boosted attack. I can use the Ion Cannon to setup easy shots for my TIEs. The Seismic Charge is brilliant. I can do damage without rolling dice or using an action.
For me the Firespray plays like a large X-Wing, with double the toughness and an Ion Cannon to setup the rest of my ships. There is nothing wrong with that.