Clue us in of what drive train you have and tire size. Are you sure the 15" wheels are not interfering with the calipers without the spacer. And are the wheels and calipers factory or after market? You shouldn't have problems with 15" rims. That doesn't seem right, unless the backspace on the wheel is deep.
There's a couple of things to consider when getting springs, ride height and spring rate. With progressive rate springs that becomes variable on the amount of weight placed on the spring. Being in the front that shouldn't be a concern since your not using the engine compartment as a trunk. Progressive rate coil springs were brought about for rear applications for weight changes in the trunk and towing. I can't see a practical use for progressive coils in the front, unless you're going racing.
It sounds like you're more concerned with ride height. With your current springs, are you having clearance issues? In that case I'd focus on the wheels first, especially if your using spacers.
