Might as well condense Venus' atmosphere and transport that to Mars. Two terraformings for the price of one.
Comets are around 5% CO2. At least you want the water, too. If I'm reading the paper right, you need 3.6e18 tons of CO2 for a 1 bar atmosphere -- enough to allow liquid water. Halley's comet masses 2.2e14 kg, and at a quick glance seems not terribly atypical. That means 15,000 comets slamming into the surface, if they were pure CO2, or if we can count the water and CO in the atmosphere. Or just multiply by 20 to stick with that 5% number. If we can divert 15,000 comets into Mars, we can divert 300,000. On the bright side, you won't have to worry about those elaborate clean-room procedures to avoid space probes contaminating the surface with Earthly life.
Maybe processing the entire surface of Mars to a depth of 200 meters to decompose all the carbonate minerals isn't such a bad plan after all. Nanobots, self-assemble!
Here's a
link to the paper at Colorado, and thus not behind the Nature paywall linked in the NASA article.