My 03 GT had coilovers in it when I bought it and it rode horribly. Maybe if I had adjusted them some it would have been better, but any improvement would have been marginal at best. Between the coilovers, frame stiffening and the panhard bar, the car could out-corner the tires that were on it. Found that out the hard way. No, I didn't wreck it, but I needed a new pair of jockey shorts...
It's all in the combo. Suspension components need to work together. For a coilover setup to be effective, you need to have a matched spring and shock combo. If the shock does not have enough dampening to handle the spring stiffness, it will ride and handle like crap.
When a spring and shock combo is matched, the ride and handling a very good. Much better then a modified McPherson setup like the Fox body setup.
the reason a coilover rides better is because of the wheel weight properties. With a coilover you can have a softer spring than compared to the standard lowering spring, and achieve better handling and ride quality. I am no expert in this field, but look at Maximum Motorsports website for a better explanation.
But maybe before you swap combo's you can switch out shocks or springs and get a great handling/riding setup.
I think that is your issue right there. What spring rate?
More than likely they are too stiff for your shocks. I would seriously consider new shocks before swapping springs. Even if you went back to a regular lowering spring, with a stock shock it will not get you the results you are hoping for.
Once you determine the spring rate, look into better shocks!
I have Bilsteins on my MM coilovers and I absolutley love them. The ride is great. Under normal driving, the ride is firm, but not jarring at all. But when I want to toss it around a little, they handle really nice.
But it all depends on what you like. Koni's are nice because you can adjust the setting from soft to firm. I went Bilstein because there is no fuss. It adjusts to your driving style.
They are both worth the money when matched to the spring rate.