The velocity loss between 26 and 24 inch barrell is probably around 60 FPS. See the attached link which compares velocities from 22 inches down to 14 inches. Pleasenote these are from the same barrel being shortened, I feel this is the only way to accurately judge velocity loses.
http://www.accuratereloading.com/223sb.html
I would stick with the 223 for varmint hunting the ammo is cheap and it shoots plenty flat at 3200 fps.