"I wanted to get the 3.73 rear axle ratio and was wondering if it was worth it."
It'll cost about $600 to have a 3.73 ring and pinion swapped in. It's not something you can do yourself.
"How much faster will the car go 0-60 and in general."
Hard to say. with 3.73's you have to shift into 3rd before you hit 60 MPH, so that will add a tenth or two, and negate some of the gains from 3.73's.
"Will it effect the fuel efficiency greatly?
City mileage won't be effected too much if at all, highway mileage will probably drop by 2 or 3 MPG
"Also is the C&I cold air intake worth getting."
Probably won't hurt anything, but it'll probably require a tune to get the most out of it.
You should probably be aware than you'll void the powertrain warranty if you do any of this. Ford has been really cracking down on denying warranty coverage to 2011+ Mustangs with aftermarket mods.