Best improvement in cornering power comes from good rubber. No matter what size wheel.
You can have the car leaning 'till the door jambs scrape asphalt, if you're running good rubber the car will just corner.
Doesn't mean it will feel good, and certainly won't be pretty, but it works!
I don't know about those rubber spring inserts.. That reminds me of old Nascar, where you could see the crew chief pushing wood wedges in between the spring coils to change the oversteer / understeer balance. Those things will change your effective spring rate, but nothing comes close to a set of good struts. Even on stock springs.
Does that help?
