I would guess for a turbo at 6 to 8 psi, you would be looking at 60-70 horsepower gain, but there is a lot of money and work to go into turbo'ing an N/A car, tunes, electronic sensors, lots of things like that, plus the basic cost of the turbo kit and the cost of dyno time. There are a few on here that are running turbos, I would contact them, they could give you a lot better details about it.
The benifits of the K&N is better air and more of it, but I doubt you will feel any difference, but it might get you 1 mpg in mileage, if you really want to add some power, get a nice cold air intake, it will be kinda pricey, but its worth it, adds to the feel of the car and got me almost 4 mpg.