somewhere between 0 and 100% worse.
They do extensive aero testing on all cars and I remember them saying that the rebel was actually more aerodynamic (with the new front end) than the regular ram which is part of why it gets the same gas mileage as the regular rams with more weight (std air suspention vs non, skid plates and steel bumpers) and much more aggressive tires. Most cars now have an air **** or "smoothing plate" on the front bottom to help create a slip stream going over and under the car, who knows if this may push air through the engine bay and disrupt that. Cars are essentially a reverse airplane wing and it creates "suction" or downforce as the car moves, if you add another hold to pull air though it MIGHT reduce this effect, like having a hole in your vacuums hose.
If you have your own wind tunnel you might be able to get a rough idea by trying it out your self. Other than that there is no real way to know. I doubt it will do much of anything at all other than dirty your engine bay, that area of the hood and windshield is very very low pressure by design. It makes it so most things get blown over your car instead of straight into your windshield and makes a bubble of air for the pressurized air to flow around. The bed of a truck is also designed to do this, you can see it in the pic, its also why your tailgate has a spoiler (wide top) to smooth out the air as it leaves the cars "bubble" and not cause a cavity of suction behind it as the car moves through the air.