Ships & Modules

Would having damage mods in your low slots increase the damage of the orbital strike? Or is it a fixed amount of damage it can do?

am i the only one who doubts the destructive power of an orbital strike launched from a 125mm cannon? hint: WW2 artillery shells usually had 150-225mm caliber and their damage potential was not quite large enough to be seen from orbit....

You're shooting with giant guns from outer space, not with this:
http://guadalcanal.homestead.com/files/Jap_150mm_gun_Honiara.jpg

To answer OP's question, the more turrets you use, the more damage you make. Destroyers with their 7 guns are quite good at orbital bombing.

WW2 Artillery shells didn't contain antimatter and were dropped from orbit at 0.X% of light speed.

EDIT: That said the easiest and by far cheapest way to glass a planet would be to but a tractor beam on Your ship and pull some close by, large enough asteroids into the gravity well of the planet. or just fill Your cargo Hold with large chunks of metall and drop 'em. The forces acting on impact should equal that of our modern day nuclear weapons.

That thought always ruins the immersion for too. It seems to me that one small hole in the hull would tear a ftl speed ship to shreads.

Not by a long way...

An asteroid used as a planetary bombardment munition is a kinetic energy impactor (with perhaps a small chemical payload) - so if you accelerate it towards the planet with a KE of (for example) 150MJ it will have the same energy on impact as any other projectile launched at 150MJ...

Except for the fact that the composition of an asteroid is unpredictable and its structure inconsistent - any atmosphere between launch and impact may cause significant issue both in targetting and projectile break-up (particularly with pcokets of solid CO2 or H2O which might vent in all manner of different directions) not to mention the inconsistent aerodynamic profile.
Compare that with a tungsten penetrator with a hyrdodynamic design*; consistent density, no materials to sudlimate out of the core, a constant aerodynamic profile...
By the time it's reached the target a manufactured projectile will have lost less mass, be closer to its target (and stands a much greater chance of actually reaching the ground)... and it fits relatively standard weapon systems which do not require significant retrofitting to mount on a standard hull.

Then you have the transport costs - yes, your manufactured munitions have to be transported to the bombardment site, but so do your asteroids, you aren't going to find them inside geostationary orbit, if you wanted to bombard Earth you're looking at a trip well in excess of a year in each direction at sublight speeds - and each trip would provide a few (in EVE terms a maximum of eight) rounds of ammunition.
In order to accelerate these towards the planet you have the issue of your launch platform; either you need to ship or construct a specialised launch platform (which is likely to mean far more mass to ship than just a crate of ammunition) or you’re going to have to use a ship to drive the ‘roid towards the target, either by tractoring it behind your vessel (what are the losses from the tractor beam itself?) or by putting the nose of your vessel against it and pushing it (probably requiring reinforcement to the nose and certainly requiring repainting afterwards). The first and last of these methods would also require work by a bunch of guys in space suits to remove lumps and bumps which might damage the launch equipment.

You also need to consider the longer term effects of your bombardment – You’re presumably intending to occupy the planet after you’ve conquered it, so the time it took for the ecosystem to recover after the KT impact would be unacceptable, a more surgical strike is possible with more conventional weapons.

Finally... If, instead of using a kinetic impacter you use a projectile which includes a stored energy element (an explosive for example) then the energy you need to invest at the point of launch can be significantly reduced. The explosive yield adds to the kinetic yield, and with a ten or so grams of antimatter (held in a magnetic ‘bottle’ until impact) that could mean energy yields equivalent to millions of tonnes of TNT...

All in all, whether you’re The Imperium of Man, The United Federation of Planets, The Covenent, or a capsuleer supporting your DUST comrades; conventional weapons are significantly more valuable, more efficient, more effective and even cheaper than using mass drivers to pound it with asteroids.
On the other hand (as with the Centari bombing Narn back to the stone age) the dust, climate and other global effects of asteroid bombardment might have a beneficial morale effect if you’re willing to pay for it (but that’s a big bill, both in time and money).

except something that fast and that small will never hit the ground?

Railguns aim to fire at 3km/s and can use shaped, shielded slugs. Also you can give the slugs a hellova lot more mass