Maybe you could keep the springs you've got and get a set custom wound to suit at the back by any spring manufacturer you like.
A standard linear MacPherson strut coil at any rate adn length you like shouldn't cost more than $200-$250.
Note: Many spring manufacturers (for my car anyway) wind quite modest off-the-shelf spring rates, even in 'sports' or 'low' configurations. These springs will work thus adequately with OEM dampers but not increase spring rate significantly.
For example, the only two linear rate coils I could find off-the-shelf for my TA22 were Dobinson (~90lb) and Lovells (~115/105lb) The car will go on the road with these Lovels coils and modest dampers as a baseline before further revisions are made. Ultimately I think I could approach spring rates of around 140+ for regular street use with weekend track use.