Also, would the temperature really go down and not be negated by the kinetic impact?
Apart from that, more water probably helps, if we do not need that water more urgently on mars.
Then there is also https://en.wikipedia.org/wiki/Oort_cloud
Don't worry - there is more than enough comets to bomb the Mars with all the future settlements. Muhaha
The ice comet solution has the added benefit of wiping out all of those pesky Martians, given the kinetic energies involved. Just ask any Pak Protector. (-:
There is no H on Venus surface, so it is impossible to change Venus with local resources. So, they have to be imported - and there is H in comets. So, the only way to teraform Venus is with bombarding it with thousands of comets and space junk, that contains ice. Maybe getting H out of gas planets might be another source, too.