The driveshaft wouldn't be a "waste" because it does more then just speed. It improves acceleration, fuel economy and overall handling of the car.Unless its important for you to drive at speeds of 140 plus, waste of money for a new driveshaft as one of your first mods. One can clearly drive safe doing 120 MPH with the stock driveshaft on an enclosed track of course. Never on the public streets
A first must have mod is, sct bama performance tuner along 3:55 or 3:73 gears. Our stock 2.73 gears are horrible. That's around your budget.
CAI'S are great but you will only notice considerable gains when combined with aftermarket headers along with off road x pipe or high flow cattwd x pipe. That's down the road for ya.