Willy bought these from us.
You still may not like the ride but you really should install the rear springs. They are made to work as a set. If anything, you are throwing off the handling of the car by only running the fronts. Also these springs are progressive as stated by H&R's themselves. It's listed right on their website.
"Sport Spring kits are H&R’s number one selling product. Enhance the look of your vehicle with a reduced fender well gap. H&R Sport Springs lower the vehicle center of gravity and reduce body roll for better handling. The progressive spring rate design provides superb ride quality and comfort. A lower wind resistance signature will make the vehicle more streamlined and improve gas mileage. If you are only looking to improve one part of your vehicle’s suspension, you cannot go wrong with installing Sport Springs. Fun to drive, H&R Sport Springs are the number one upgrade for your vehicle."
These springs are listed for the 2013+ MKZ and in conversation with H&R they stated that they indeed were tested on the MKZ before being listed so I wouldn't be so quick to write them off as being matched improperly. Anyway like I said, you still might not like them but I just wanted to chime in and provide additional information. The wheel looking pulled in is going to be something that is more noticeable on most lowered vehicles with bigger wheels. Spacers or aftermarket wheels with a slightly more aggressive offset might fix this.