Gravity is a hardcoded constant acceleration of 9.8m/s downwards. There is no air resistance / friction in SL (atleast.. not for physics objects, avatars is a whole different story). For llRezObject, there is a cap on the velocity magnitude at around 250m/s. I think this doesn't apply to objects gaining speed by falling from great height (or by llSetForce for that matter), but I am not entirely sure.
If you want to adjust gravity, you could always change the gravitational force on the object with llSetBuoyancy(float buoyancy). This function is equivalent to an additional force which counter-acts gravity: llSetForce(<0,0,9.8 * buoyancy * llGetMass()>, FALSE). Setting buoyancy to 0 will not affect it, setting it to 1 will negate gravity completely. Note that this function still counts towards prim energy.
Any projectile in SL should describe a more-or-less perfect parabolic arc. This will be roughly equivalent to the arc a cannonball for example might make in a vacuum. In RL, objects with a low density (say, a soccer ball), will tend to describe a different trajectory, mostly due to air resistance. It won't be much different from a parabolic arc though, maybe a bit shorter. I think the resulting trajectory can be approximated by just having a smaller launch velocity.
If you want to simulate the launch of a projectile which has complex areodynamic properties (wings), you might want to script the projectile itself to constantly update a llSetForce correction or something similar. This can get really difficult to get right though.