Well computers aren't useless on a missile boat, they raise initiative. There are some points where initiative is so important that people lacking a good computer will mount augmented engines... but since I can't remember the details, I'll shut up here.
"No need" =! "useless". There is no single useless tech in a whole game (talking bout Tactical combat setting), while total majority of them at the same time are not needed at all (unless you just want to use them "just for a sake of using it").
Actually its about some special cases of possible non-mounting of Augments in a certain cases, reversely. As Augments is not only about initiative, but also combat speed (thus maneuverability and beam defence), making them the must-be ship equipment, beside the aforementioned special cases (talking about basic shipbuilding here, not "but i can build a ship without it, and still pwn AI" stuff, as its 2 different things).
I've played on Impossible since release. No need for a computer on a missile ship as I'm pretty sure it only aids beam targeting. Lots of games (small galaxy size) can be won with nothing better than a few ships with 2x MIRV Merculite or Pulson missiles with ECCM from a sub-Titan ship with Reinforced Armor. If you want to take down Orion all you need is a few of these ships and then you can refit with the goodies you get from Orion or from Loknar's ship. I don't have much trouble with missiles being stopped when they have ECCM, Fast and Heavy Armor but if so then the first rack would be of a lesser vanilla missile type. Fast missile racks can also make nearly impossible battles go your way as you can launch everything at once.
The risk with missile ships is you can destroy much or all of a planet with overkill when you try to take out planetary missile bases and other defenses. If the AI only has minimal armor then a MIRV of Pulsons could obliterate everything, including the population you want to pacify to boost your score.
You dont need even such low tech to win a game, actually, it could be won without any research at all, using only the tech given via Average tech level (basic nukes mostly). For Orion you need just 9 destroyers, also.
The best beam weapon for it's size is the Plasma Cannon, I always use the heavy version for greater range and power. It hits all four shields meaning quadruple damage and if you add in High Energy Focus, Structural Analyzer or Achilles Targeting, it doesn't take many cannons to destroy any enemy ship, even Antarans. I don't often bother with shields as it takes up room I could use for weapons and with a good computer my move is usually first meaning they don't get to fire at all most of the time. Shields don't work against Antaran weapons either.
Best for [maximum possible damage/size] ratio, correct, not necessarily "the best beam weapon" thought. Shields are very good stuff (despite they are indeed somewhat weak in balance in vanilla moo2), and they do work against Antaran weapons (except 2 of them, with 1 tech's addition decreasing the number to just 1). Not what Antaran weapons are of some real deal, just to clarify. One of most valuable effect of shield is ability to counter Transporters, as ship's lifespan in a battle involving them is strictly depends on a shield arc is running well (not a deal VS AI ofc, as AI is not actually acting in a battles).
If I want to beat the Antarans I always take Orion, sometimes using a saved game until I get the tech I need which is at least Damper Field. Xentronium Armor is also nice but Damper Field cuts all incoming damage by 3/4 which means even Neutronium Armor is adequate if you add in Reinforced Hull/Heavy Armor.
There is absolutely no problem with beating them on a standart Zortium armor (or even Tritanium, as its only about the size of fleet needed for a task there), as there is little sence to move further in this techline after one got a Zort. Heavy armor is a quite weak technology, and, while have some possible usage is not necessary at all (especially if you're bother about ship space saving to increase a payload so much, so drop a shields), ideally one prefer to make one's ship not getting a hit at all, not making it harder to go down by adding HA. Its about speed and initiative here, not HP bloating (while it work too, and needed too, to a some extent).