Have any of you experienced Back/Front focussing with Nikon 17-55/2.8 Lens? Need your advice

All, i currently have Sigma 17-50 OS lens and using it on D7000.. there is no problem while using it in day time.. but in the low light it hunts badly and i miss some shots of my kid when we go out in evening. Performance of Sigma 17-50 is really good, it takes sharp images, focuses very fast but in the low light it is not a good contender.

as i m comfortable with the focal length i prefer to go for Nikon 17-55/2.8G lens...which will give me advantage of focussing in low light situation. correct me if i m wrong.

May sell the sigma and acquire an used Nikon 17-55 lens with little top up.

Your valuable advice is appreciated.