Do the cometary gas ions which make up the plasma tail of a comet reach the velocity of the Sun's solar wind?
Yes, the ions are said to be "swept out" by the solar wind, such that they reach the speed of the solar wind. That speed varies a fair amount, however, because there is both a "fast" (about 600 km/s) and "slow" (about 300 km/s) solar wind. It is really the magnetic fields entrained in the wind that sweep out the charged ions, since each ion has such a low mass it is easily affected by those fields. Note the dust particles from a comet, which is the tail that we see, are much more massive and are not affected by the solar wind, so they are moving out much more slowly and often make a noticeably curved shape.