Firstly you mention you know coilovers are stiffer, well they're not, whether it's a coilover or not doesn't change the spring or damping rates. Secondly there isn't enough clearance to fit a proper size damper in there with then a spring over, so manufacturers use a much narrower damper to fit it all sacrificing control. Thirdly, stiffer doesn't always mean faster, if the setup is too stiff you'll have a much earlier loss of traction. If i could supply a rear coilover that bolts in and still keep the performance there i would.