Dave has a good - tire upgrade is the best bang for your buck mod you can do to almost any car. That Hotchkis will help, if indeed, the car had some chAssis issues giving you that funny highway behavior which may not be helped with a tire switch.
You are correct that most manufactures don't list spring rates for their components. The fact that you are even aware of spring rates - means 1. You are smart and 2. You'll least likely to get screwed. That's why lots of springs and coil-overs end up on eBay and such - people get them because it would be "cool" and end up having the car ride like $hit.
Those spring rates sound about right. Being a FWD car - increasing the spring rate up front will increase understeer, increasing the tendancy of the car to push in turns. A good thing for a FWD - can't really steer out of a spin as well as a RWD. Progressive rates are recommended for soem FWD because of the weight distribution and stiffness of the chAssis subframe.
If you want to go with aftermarket springs - most will lower the car - you will have to get new struts because the OEM cannot handled the reduced ride height. Usually why some are sold as a set - the struts have a shorter body and valved differently for the revised spring rates.
I'd try the Hotchkis kit when you get it to see if the ride gets better. This assumes that the alignment and tire quality are all good. Adjustable struts may be the better answer - you can dial in more or less valving, depending on your tastes. Upside is you generally can keep the OEM springs and ride height - with the relatively low miles you have, I doubt they are shot.