Putting GC coilover sleeves on Koni shocks effectively creates a "full" coilover. The spring perch of the Koni shock is totally removed and replaced by the GC sleeve. The shortened shock works well with the lower-than-stock ride height, because it will gain back some suspension travel. I can't say the ride will be close to stock but it won't be intolerable.

As for coilovers versus shocks and springs, the only advantage performance wise that coilovers provide you is the ability to adjust your corner weights. Unless you put the car on a set of scales and dial in the corner weights they won't really do anything for you. However if you do have optimized corner weights that will provide you with more balanced handling regardless of which direction you're turning, instead of with plain springs which tend to make one direction better than the other (this is the case with stock springs as well).