Would another front facing gun on the roof make it too powerful? Too similar to the spire?
Too similar/powerful probably.
Junker and Galleon - 2 guns on each side, plus a gun facing along the axis of movement (plus the galleon gets a side light weapon).
Pyramidion and Mobula - Strong forward facing arcs combined with poor maneuverability and large blindspots.
Squid and Goldfish - Featured front gun with 2 more guns at 90 degree angles from each other. High maneuverability with low damage output.
Spire - (when not dead) Strong arc overlap with no durability
Junkers and galleons get good arc overlaps, and thus have seen more use in competitive play. Good gunning/engineering can help overcome piloting deficiencies. Good communication ships.
Pyramidions and mobulas get good arc overlap, and thus have seen more use in competitive play. Pilot awareness is key to protect blindspots.
Spires -
Opponent has a merc/hades/gatling? gg (not really, but spire is the weakest ship IMO)
Squids and goldfish have poor arc overlap and thus have seen less use in competitive play. Ability of pilot to blend all weapons into use regularly is a must, and engineering oversights can leave you drifting.
Squids and goldfish properly maintained have amazing durability, and can enter battles and disengage better than any other ship. Lacking is the ability to get easy overlap of guns. It requires much higher pilot focus to blend together 2 guns at the angles used. Change the angles and make overlap easier and you get ships that can easily engage and disengage, and also get kills quickly without requiring the pilot to be perfectly dialed in.
I consider the squid to be the pilot's biggest challenge, and the goldfish to be the ship needing the highest level of synergy between gunner and pilot. I know this isn't about the squid, but it does come up. Watching Puppy Fur fly a squid makes me feel like a hack who gets by on just a couple tricks. The same can really be true about a goldfish. A goldfish is really sort of a heavy squid - as Nidh says, a ship that gives its ally an advantage. We already have ships with strong arc overlap. A well flown squid or goldfish are much more interesting to see fly, simply because it takes a lot more creativity to blend in different damage types.
TL;DR: Goldfish can be effective, just not in the same way as other ships. Giving it easier overlap makes it more boring. It simply requires more skill and dedication to be truly effective, and more trust in your ally.