Okay, okay, here's the magic formula:

STI = ((((3*#s)+(5*#l))(A/1000))/(Ta*Tr*(S/10))*C

#s = number of small weapons ports
#l = number of large weapons ports
A = armor points
Ta = angular time (taken to turn 180 deg w/light engine)
Tr = rectangular time (taken to go from 0 - 60 in a heavy engine, no boosting)
S = size (in meters)

C = normalizing factor = 0.485

Eldrad: I agree that a 7k hull Valk with 2s weapon ports does seem a little drastic, perhaps the formula needs tweaking, but no way is the current Vulture fairly close to the current Valkyrie. I'm sure that would be true if you were particularly adept at the Vulture and going up against some scrub in a Valkyrie, but if two *equally* skilled pilots squared off, one in a Valk and one in a Vult, I would give pretty good odds to the Valk (8:3 to be exact ;). It has more armor, 50% more firepower, and is noticably more maneuverable than the Vulture.

Daon: Ceteris paribus if the cent & the valk swapped maneuverability the Valkyrie would have an STI = 5.09 (just below the current Maud) and the Centurion would have an STI = 2.08.

If the Marauder were brought down to 2s weapon ports it would have an STI = 3.68

Physical size doesn't really matter when it comes to the valk because the effective distance of energy weapons is about 80m at this distance a valk can easily dodge anything and in close quarters the valk has a more hull, has an extra weapon slot and the vults flat shape makes no difference at 20m.

I think the marauder's maneuverability is fine at the moment but maybe they could remove one weapon slot and increase the hull seeing as it's a trading ship not a combat vessel.

Ta has way to little effect in your STI equation. A rag with the the turning capabilities of a vult would score a 3.65 making it a little better than the current prom, or vult. But with dual gats it could cream anything that comes at it... so here's another idea... I haven't really worked the kinks out of it so it will need some adjusting.
Also size is not a good measure of difficulty to hit. The vults is the hardest to hit (with energy weapons) because it's so thin if it's looking at you (this can be over come by broad siding it). Since I don't really know the size of head on cross sections I'm not going to include them

Ship Defence Index:
SDI = (A/1000) / (S) / (Tr^2)*C

Ship Offence Index:
SOI = (#s*(4.15-Ta)^2 + #l*(4.2-Ta)^3) * K


Where the variables are all the same as Phaserlight's, except C and K which are both normalizing constants which have some arbitrary values. All the coefecients need to be tweaked, and you could probably break SDI into SDI(rockets/homers) and SDI(energy). (Rockets/homers) would be effected only by Tr^3 and A, where as (energy) would be effected by crossectional area as well as Tr^2 and A.

Ok... I've actually looked at this for a number of ships and will edit it to include all ships in a while.

I don't mean to be frustrating, but shouldn't the number of weapon ports be additive rather than exponential? I don't think having 2 l-ports is 8 times better than having 1 l-port. (edit: oh, I see.... it makes sense now: #l*((4.2-Ta)^3) not (#l*(4.2-Ta))^3. A big difference in your formula is that you have s and l ports as coefficients of separate terms whereas I lumped them together as overall firepower. Anywho I'll be interested to see how the numbers come out ;).

Also, in the Defense formula do you mean ((A/1000)/((S)(Tr^2)))*C? Just wondering about the order of operations...

Edit: oh yeah, I don't think a rag with the turning capability of a vult would beat a valkyrie. The rag is just so huge with relatively little armor, and it still wouldn't be able to dodge much. Now a rag with the *accelleration* of a vult on the other hand.... stick that in your pipe and smoke it ;) *shudder*

An adv gat that you couldn't ever get away from would be awful. Anyways here's the list.

Name SDI SOI
Valk 16.76 13.36
Vult 7.77* 9.16
Cent 5.19 4.54
Rag 0.84 5.32
Bus 3.47 4.20
Marauder 5.64 11.52
Hog 3.64 11.96
Hornet 2.80 12.11
Prom 3.64 6.06
Wraith 2.26 9.76
Atlas 1.25 7.17
Centaur 0.89 4.89

* I would multiply this by 1.5 because of the extra thin profile, giving the vult a SDI of 11.65

Comments:
The Ta does not give a large enough difference in ship turning accel (because I think all ships have the same max angular velocity). At some point I will test the time it takes for a ship to turn pi/6 and then switch turning directions until it is facing it's origninal direction. This would give more varried results since the ships won't be turning at max speed for as long (or at all maybe).
After I do that I will retweak the formula basically so the hornet's SOI drops since it has relatively bad turning and no large ports.

Here's some more balance idea's :

Give the centaur 18000 hull.
Give the ragnorak 24000 hull.
Give the hornet 12000 hull.
Reduce Valk's maneuverability to the vults level.
Give the large gatling (not the adv one) cannon a narrower spread.
Increase the damage of the large plasma cannon.

That's all for now.