What would be the impact, given adequate technology, of redirecting large ice-rich comets to hit mars?

Interesting question. It could, conceivably produce a water rich environment.

After all, theory suggests that that is how the earth gained it's water. Mars, however

lacks the atmospheric pressure to long sustain liquid water.

For Mars -- no change.

Mars has 0.532 the diameter of earth (roughly 6,000 km), a mass 11% of earth's and surface gravity of 0.37 g.

An impact by a 15 km diameter comet won't change any of that. Even adding 1000 such comets would not have a significant effect.

The problem is the surface gravity. Any volatile gases would be lost to space because of the low gravity.

And there has been a thousand impacts on Mars in the past. But Mars still has virtually no atmosphere.

If each comet averaged a cubic kilometer in volume, it would take a quarter-billion comets to get the same ocean-to-planet ratio as Earth.

Given adequate technology, I think it would be disastrous because colonies will likely be established on the planet before entire oceans are sent, since technology would be good enough to sustain a self-contained colony for a while. By then, Mars might be used as a half-way station to Jupiter's moons, which already have water available for harvesting. Any comets striking the planet could jeopardize our ability to send supplies to the Jovian colonies.

You underestimate scale. The planets were formed from the asteroids and comets in the solar system and are stable now because almost all the asteroids and comets were used to form the planets. The mass of the entire asteroid belt is less than 4% of the mass of our Moon which in itself only 1.2% the mass of Earth. You would have to go incredibly far to find enough comets to terra-form Mars, there simply isn't enough unused material in the solar system to terra-form Mars.

Also, it's totally un-necessary to terra-form Mars. We can build O'Neill Cylinders, Bernal Spheres and Stanford Torus's to colonize. There's enough material in the Asteroid belt to house a quadrillion people in such colonies. Unlike Mars which would always have low gravity leading to deformities in the children growing there, a space habitat can have a full 1-g gravity by rotation. Large structures are easy to build in zero-g where weight need not be supported till the structure is complete. There are no planetary gravity wells to launch against and no atmospheric re-entry to worry about. With propulsion, space habitats become generation starships.

The difference between building an ecosystem on a space habitat versus building one on Mars is like the difference between mowing your own lawn versus mowing everyone's lawn as well as weed their gardens and walk their dogs.

It's only a planetary bias and a lack of imagination that we only consider terra-forming and colonizing planets and moons. Open your horizons and you will find that many more options are possible for less effort if you just let go of the obsession of colonizing planets and moons and colonize space instead.

You would have more ice on Mars, at least for a while, until it sublimated and went back into space. Mars is too cold for ice to melt, and doesn't have the gravity to hold onto an atmosphere that might contain water vapor. Maybe there was liquid water once on Mars but it was millions or billions of years ago, when it was warmer. Since then it has obviously evaporated into space.