No it isn't overcosted at 355. I'd be happy to play with it at 355pts any day of the week. So would Sigoroth. I'd entertain the idea that it's overcosted in an absolute sense, but by your own admission the Smotherman Formula is only a rough tool - a 10pt difference from Smotherman is not ironclad proof of overcost! Relative to the other IN battleships it certainly isn't overcosted:
Compare to the the Apocalypse:
+ More powerful prow armament(10pts)*, speed 20cm(10pts)**, significantly more powerful dorsal armament(10pts), doesn't lose shield/engine power when firing at range(10pts)***.
- 12lances@60cm for 36WB@45cm(50pts).
Retribution 10pts fewer - gives 355pts.
Compare to the Oberon (which is going to 345):
+Armoured Prow(35pts), Speed 20cm(10pts), significantly more powerful prow armament(15pts)****, significantly more powerful Dorsal Armament(20pts)
-1 Turret(10pts)*****, Sensor Array(25pts), 1/3 broadside exchanged for LBs(15pts), 1/3 broadside exchanged for 60cm lances(15pts), 1/3 broadside range upgrade(10pts).
Retribution 5pts more - gives 350pts. Within 5pts is close enough.
Compare to the Emperor (still 365)
+Armoured Prow(35pts), Speed 20cm(10pts), significantly more powerful prow armament(15pts), significantly more powerful Dorsal Armament(20pts)
-Sensor Array(25pts), 2/3 broadside exchanged for LBs(50pts)******, 1/3 broadside range upgrade(10pts).
Retribution 5pts fewer - gives 360pts. Within 5pts is close enough.
Departures from Smotherman in calculating the above:
*As playtested all-NC fleets do objectively equivalent or worse to torpedo fleets, the NC objectively can't be worth more than 6 torps - 3 extra torps are worth about 10pts.
**20cm speed is objectively worth more than 1pt. I'd say it's normally worth at least 5pts, but to a 15cm battleship 5cm speed is the difference between needing a special order to turn whilst under fire or cippled or not. 10pts is fair, and i'd even be tempted to value it more.
***Obviously the value of special rules has to be guestimated, but I think 10pts is fair.
****60cm Lances are priced 18% more than 45cm Lances. As the only difference is the range, this can be fairly applied to WBs as well. I therefore calculated using 60cm WBs at 3.6pts each.
***** The 5th turret pretty much renders the Oberon impervious to bombers - it's worth more than 5pts.
******8LBs are objectively 15% more than twice as powerful as 4LBs due to fewer casualties to turrets. Price increased by 15% to compensate. Without this modifier, the Retribution comes out MORE than an Emperor.