I just bought a 2006 GT manual, I installed the Borla Cat Back. Does anybody know what could be my ET in the 1/4 mile with just that?

Also Ive heard of people doing an OEM air intake mod for the stock filter, removing something???
Is this true?

Im new to the Mustangs, Sorry..    

this will not give you much if any at all. Check out JLT true cold air intakes with the sct tuner. I have this set up and gained 30 hp to the rear wheels.  

By the way, welcome to the mustang world! My next mod is a whipple supercharger which should be done by july. Good luck with your stang and have fun.  

Changed my mind .....pro charging.  

+1.  Different cat-back won't make a noticable difference on the 1/4 mile.  Expected ET depends on many variables - track condition, air temp, humidity, pressure, weight of car and driver, and how well the driver does his thing.  14.5 is a respectable time if you are new to drag racing I think.  I don't know what the best ever time for bone stock S197 GT is, but I think it's between 13.4 and 13.0.

As far as taking something out of the OEM air intake, that might be the carbon trap - but they stopped putting that in before the 06 model year.