Back from holidays and just following this thread.
I have been playing with the ship combat rules for T20 for some time and I agree it is way too deadly. Spinal mounts are almost(?) always guaranteed a hit on any ship and the crit range is too high. Did anyone catch that "if spinal mount increase threat range by 5" rule? Meson spinals crit on a 10!?
Average roll to hit with a spinal, lets say a Type N, is 10(skill of gunner)+22(USP)+9(Successful sensor lock)+5(Succesful Command) or a 46!
This assumes a 6th level gunner with 9 ranks of gunnery and a stat of 12 for the +1. The Commander has a total leadership skill of 15 and will always make the check.
The way the rules are, a spinal gun is devestating at nominal range hitting ANY target, and can still hit and destroy ships by sniping away at far, far range!
Assume the gunner has Heavy Metal and the nominal range is 150,000km(not that -2 to hit realy matter. "Hey cap'n, watch me blast that Tigress in orbit around Pluto!" The reduce damage per range band takes care of this (more later).
I've done some home rule brewing to suit my taste in combat (those big ships shouldn't just light up so easily like that!)
1. Ignore the +5 for spinals rule. 16d damage is enough as it is. Meson spinals threat/crit on a 15 not 10. Even then this may be too high.
2. Degrade Weapon damage by 1 dice per range band.
That means meson spinals have a max range of 16 range bands, but even at half that, a meson screen will soak the dice. (See house rule #3 below) Total range 16x75Kkm is 1.2Million Km(80hex)
3. Double the AR of Meson Screens to make them more effective. Meson Screen 9 = Meson AR18. 16d20-18 = 1d20-3. Painful(usualy a 17SI), but not debilitating if the shot penetrates.
4. There should be some form of protection from PAW besides just Armour. I've made Sandcasters able to absorb some PAW fire, but at 1/2AR. Reason? PAWs are messed up by atmospheres, so why not the crystals in sand?
5. Bring back the USP v. dt Rule from HG.
Before I do any SI damage calculations, I roll x amount of rolls on the "Special" internal damage subtable based on the Weapon USP - Ship displacement number from HG. The "Special" chart is exactly the same as the criticals from HG. Subtract 1 roll for every 2AR, as per HG. Subtract 1 roll for every range band beyond the first. I do this becasue I am lazy and want to see if the ship survives to justify an SI roll. if it blows up, why bother rolling SI?

Also if a critical is rolled, and the SI done doesn't frag the ship, I roll on the special table instead of normal internals. Spinals get the +40 on this table!
It is very possible to roll a cripling hit on this chart and still roll crappy SI. Handwave for this: Your shot was so good that it fragged the system but left the hull and structural supports relatively intact.
Just some of my ideas and house rules. As always YMMV.