My ballistics program is TU at the moment, but usually you will find the initial launch speed is more important than anything when you are only shooting 400 yards or so. It's only past that, where the high BC slower bullets start to take over in the bullet drop department. Out to 400 yards or so, the 20 should be extremely flat. That 4300fps launch speed from the 20 should prove very flat and have less bullet drop than the 17 or the 223 out to 400 or so yards. In the wind, time of flight is fairly critical and that 4300fps speed should again help some, but you only have a 33gr bullet, and that is working against you so wind will still be an issue.
For varmints out to 400 yards, without wind, the 20 should fare well. If the distance is past 400 and you have some wind, I would go with a .338 Lapua or the big 50.
The local shop here has the Hornady 20 caliber bullets on the shelf, so availability should not be a problem. The real disadvantage I see with the 20, is simply the lack of range in bullet choices. Right now, there is only 30-36gr weights whereas the .17 has 15gr to 37gr and the .223 has 40-80 or so. That limits the application of the new 20 to a narrower window of application than the .223.
The bolt action varmint guys have been working with the new 20 for quite some time. You can see more info at www.woodchuckden.com