I think 15th Jan 2010 will never come again. Am I right?
As per my opinion you don't need to worry about. A good reputable college with professional staff will get only ONE or TWO day to issue you CAS number.
As visa expiry date is 15th Jan'11, organize your application schedule as per following:
>First of all give IELTS with peace of mind on On 11th of Dec 2010.
>Than prepare every thing for visa extension application.i.e arrange all documents for college, arrange supporting documents for visa application, get bank statments ready, fill up the form as much as you can. bla bla.
>On 23rd of Dec, collect your IELTS result go to the college and apply for admission and CAS. Most probably they will process your application same day or next day.
>If they issue you CAS same day send your application on 24th of Dec.
>If they issue you CAS next day send you application on 2nd of Jan'11.
In any case, if they issue you CAS on 2nd Jan'11 its still not late. Just make a deadline of 5th Jan'11 for application.