Finally, a chance to make the quote that comes to mind every time I see this thread: "A foolish consistency is the hobgoblin of little minds." - Emerson.
With that well worn proviso in mind, lets try to dredge up some continuity in the messy world of shield, gun and armor stats.
The gun firepower on fighters didn't increase drastically until 2669. If you will observe the shield and armor specs on WC2 fighters, they are roughly the same as those from the WC1 era. In 2669, everything got a massive armor increase, this is due to the weapon power increase.
For example
A laser from 2654 did 1.8 cm of damage. The Meson shields of a capital ship were typically 30/30 cm. So repeated bombardment even by the laser would ensure that eventually it would out-time the recharge rate of the shield and begin to strike the hull.
Here's the problem: you're reading it wrong... which is to say that you're looking at Victory Streak and reading what should have been written rather than what's actually there.
Rather: there is absolutely no proof that there has ever been a shift in gun values. Victory Streak lists "Penetration: 18" for the Laser Cannon... but defines Penetration as "Armor/shield penetration expressed in tenths of a cm (0.1cm = 1 armor unit)". Rachael Coriolis' claim in her GIF, dated 2669.218, is that the laser cannon does 1.8 cm of damage, not 18 cm.
(Incidentally, 'How do we know they're different? Because they're exactly the same!' is a pretty amusing thought.)
Now, Victory Streak's 'mistake', if there is one (and I'll have that argument if someone wants) is that it calls the values for fighters "cm" instead of armor units.
An Arrow V has '80 cm' of front armor instead of '80 arrmor units'. The latter is how the game physically treats the ship: it has 80 'units' and when they're hit by a laser the game takes away 18 of them.
But clearly stated fiction, whatever its etymology, has met gameplay here, and I have to side with fiction. Even if it was wrong to have done so, Wing Commander III set a clear standard that has been followed ever since - all ships after the middle of 2669 have shield and armor numbers tenfold those before.
By 2664, the phase shields were listed as much as 120/120 cm (using the Lexington CV of 2668 as an example). The 1.8 cm damage laser could no longer out-time the recharge rate of the shield, and thus couldn't penetrate it. Capital ships recieved an armor upgrade to protect them against the weapons of other capships, but fighters retained their original protection figures for the most part.
All of the sudden, in 2669, all weapons recieved a massive boost in power. The laser now did 18 cm of damage instead of 1.8 cm, a ten-fold increase (even though the VS manual describes the weapons as doing their old damage rates, we know from the game mechanics of WC3, this just wasn't the case). Everything from fighters to Dreadnoughts recieved large armor and shield boosts. Even still, a Thunderbolt VII with full guns did something like 250 cm of damage / second. A Kilrathi CV with 2000 cm phase shields would have a recharge rate of 200 cm / second (per quadrant). One Thunderbolt VII was able to out-time the shields releatively quickly and cause direct hull damage, a situation that was similar to the WC1 era.
Although, by 2673, phase shields had again improved past the point for fighter weapons (or even most capship guns) to penetrate them.
First, let me say that I think you're using upgrades as somewhat of a crutch - not necessarily here, but it shows up in a lot of your ideas. We've trotted out "and then they upgraded the fleet!" as explanations for a whole lot of things (changes in capital ship armaments, changes in styles between games, etc.). Frankly, I don't like it - it's the "go hit home runs" style of coaching and it doesn't ever sit right with me. The image of the giant war dying down for a few weeks and everyone going home to bolt new armor onto their ships or to replace one particular turret with a new kind of gun has become somewhat of a farce in my mind.
Anyway, assuming we've established the 2669-shift in armor and shields, then I think the community's old standards for explaning them are just fine. They're wrapped whole cloth in continuity and are as beautiful as anything in their simplicity:
* The WC2 technology, "Phase Shields" is now used on fighters. This is an end-all explanation of shields; I believe it's the Wing Commander IV novel which uses the term phase shields to refer to the type found on fighters.
* Armor is now 10x, 20x or 60x (Plasteel, Tungsten and Isometal, respectively) stronger. This is explained in Privateer - the new armors are multipliers. A centimeter of Tungsten is the same thing as twenty centimeters of Durasteel. A Hornet that's armored with tungsten would, then, theoretically have 60 cm of armor equivalent instead of 3 cm. It hasn't been physically fattened, it is practically the same ship.
Now, the guns... as I said above, we cannot prove that the guns were increased from a fictional standpoint. And here's why: for all we encounter them on a daily level, gameplay mechanics can never win such an argument.
It is all well and good to take some numbers from Wing Commander III's engine and say that the Arrow has 90 'units' of armor instead of 80 cm. But what are these units? The game doesn't specify - it doesn't need to, it's not thinking about such things and it's not trying to let you see how it works.
Furthermore, the game is full of modifiers that make any attempt to compare 'in game' play to a continuous historical model useless - do they really weld on any extra 50 units of armor to Blair's fighter that they don't give anyone else? Do the Kilrathi happen to send extra-shielded fighters against Confed pilots they know are especially good, and lesser-shielded ones against Confed pilots who they know are especially lousy? The game is constantly cheating to make itself fun - taking one uncategorized number and pretending it is the end-all of debate doesn't work!
So... what do we do?
The one thing I never understood was the KS manual entry for phase shields. It stated (by 2667 I think) that phase shields offered 7,500 cm of protection. Was this for the most powerful installation shields? We certainly don't see any Confed Capships with 7500 cm phase shields and the Kilrathi Dreadnought had 8000 cm.
This touches on a very old idea, which was that 'Phase Shields' meant a single number that was so high (or that recharged sof ast) as to be practically invulnerable. The modern view is different, and it's probably best to assume that 7,500 is some kind of scientififc upper bound for phase shields.
