I think a bumpsteer kit from MM/UPR/Steeda is the way to go. however eliminating bumpsteer through correct tierod geometry would also cause a reduction in the contact patch during turns.
Agreed, sportlines lower the car past being effective for handling, their ride quality is also pretty terrible. Within a couple of months the car will be riding like one of those lowered pickup trucks you see bouncing around.
Save yourself some headaches and swap them out now. This way you don't need it aligned twice.
Most people wouldn't know what bumpsteer felt like if it hit them in the head, which it pretty much has to do to make itself evident. What people DO feel, and what a lot of people confuse with bumpsteer, is "tramlining", which is the condition where the car gets pulled around by ruts in the road and uneven pavement. These cars are terrible for it, especially with wider, lower-profile tires installed.
That said, the OP's car would look 100% better if the rear was 3/4 inch lower.