retribution plasma man is not 144dps with both broadsides lets break it down 6cros =(((4 attacks X 15 damage)/12 second reload)x6 batteries =30 dps ( it think you have done the multiplication wrong I have gone through mulitple times to be sure its important to work out dps per battery and then X by the number of battery's 1 battery has the dps of 5 (4 shots x 15 damage =60 damage/ 12 second reload= 5dps per battery X by 6 batteries 30dps from macros (15dps per side) + 3 lances with 12 damage and 12 seconds reload = +3 dps to the main target). this means the total dps excluding torps retribution can output is 33 and you can, in theory, take 3 of them ( with some points remaining) making the 3= 99dps. double cairn has a dps of 52
however i feel you are missing a real big point that getting both broadsides to fire on more than one ships is very hard and often relies on
A: the enemy having a very large fleet which the necrons with this build they will not have and if you are against a spam fleet ark dispersals is so much more effective.
B: the enemy is dumb and puts an escort on the other side of a battleship so it gets shot to pieces, sorry by my comment with this although harsh is position your fleet smarter
C: they ram between the cairns that are right next to each other, well have fun with double terror or me nocking your engines out tping behind u have shooting u to pieces
In short i am trying to get the point across that you cannot take the full dps of a broud side ship at face value as its rare to get the full use against a smart player also we have not factored in miss chance for the macros with a minimum of 20% of the macro damage deducted at close range and yea i have not taken into account torps because if you play well they will not be a major concern. in terms of ap values of weapons to a degree yes but balance that against the 100% hit of the Necrons and pretty nice crit chances especially with the 100% increase upgrade X disperse lighting ark can make you a crit machine + terror mechanics makes u pretty tough allowing you to negate number but like alot of factions you have to pick the fight for when it suits you.
There are some builds that need addressing ie 3x obsidion rose on the dark Eldar (the dps is unreal) or the boarding party's of the space marines asides from that i have been able to deal with pretty much anything even nids.
I would be happy to go against pretty much any fleet that you would like yo put up asides from the 2 i just mentioned (and Necron vs Necron as its a very boring mirror match but easily winnable with terror build)
also set cairns and the BC to frontal there is no reason for them to broadside all weapons can fire front on XD